x86: Support Intel Shadow Stack with SHSTK property

To support Intel Shadow Stack (SHSTK) in Intel Control-flow Enforcement
Technology (CET) instructions:

https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf

 #define GNU_PROPERTY_X86_FEATURE_1_SHSTK (1U << 1)

is added to GNU program properties to indicate that all executable sections
are compatible with SHSTK where return address popped from shadow stack
always matches return address popped from normal stack.

GNU_PROPERTY_X86_FEATURE_1_SHSTK is set on output only if it is set on all
relocatable inputs.

bfd/

	* elf32-i386.c (elf_i386_merge_gnu_properties): If info->shstk
	is set, turn on GNU_PROPERTY_X86_FEATURE_1_SHSTK.
	(elf_i386_link_setup_gnu_properties): If info->shstk is set,
	turn on GNU_PROPERTY_X86_FEATURE_1_IBT.
	* elf64-x86-64.c (elf_x86_64_merge_gnu_properties): If
	info->shstk is set, turn on GNU_PROPERTY_X86_FEATURE_1_SHSTK.
	(elf_x86_64_link_setup_gnu_properties): If info->shstk is set,
	turn on GNU_PROPERTY_X86_FEATURE_1_IBT.

binutils/

	* readelf.c (decode_x86_feature): Decode
	GNU_PROPERTY_X86_FEATURE_1_SHSTK.
	* testsuite/binutils-all/i386/shstk.d: New file.
	* testsuite/binutils-all/i386/shstk.s: Likewise.
	* testsuite/binutils-all/x86-64/shstk-x32.d: Likewise.
	* testsuite/binutils-all/x86-64/shstk.d: Likewise.
	* testsuite/binutils-all/x86-64/shstk.s: Likewise.

include/

	* bfdlink.h (bfd_link_info): Add shstk.
	* elf/common.h (GNU_PROPERTY_X86_FEATURE_1_SHSTK): New.

ld/

	* NEWS: Mention -z shstk and GNU_PROPERTY_X86_FEATURE_1_SHSTK.
	* emulparams/cet.sh (PARSE_AND_LIST_OPTIONS_CET): Add "-z shstk".
	(PARSE_AND_LIST_ARGS_CASE_Z_CET): Support "-z shstk".
	* ld.texinfo: Document -z shstk.
	* testsuite/ld-i386/i386.exp: Run SHSTK tests.
	* testsuite/ld-x86-64/x86-64.exp: Likewise.
	* testsuite/ld-i386/property-x86-shstk.s: New file.
	* testsuite/ld-i386/property-x86-shstk1a.d: Likewise.
	* testsuite/ld-i386/property-x86-shstk1b.d: Likewise.
	* testsuite/ld-i386/property-x86-shstk2.d: Likewise.
	* testsuite/ld-i386/property-x86-shstk3a.d: Likewise.
	* testsuite/ld-i386/property-x86-shstk3b.d: Likewise.
	* testsuite/ld-i386/property-x86-shstk4.d: Likewise.
	* testsuite/ld-i386/property-x86-shstk5.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk.s: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk1a-x32.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk1a.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk1b-x32.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk1b.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk2-x32.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk2.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk3a-x32.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk3a.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk3b-x32.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk3b.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk4-x32.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk4.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk5-x32.d: Likewise.
	* testsuite/ld-x86-64/property-x86-shstk5.d: Likewise.

binutils/testsuite/binutils-all/i386/shstk.d

@@ -0,0 +1,9 @@
+#PROG: objcopy
+#as: --32
+#objcopy:
+#readelf: -n
+
+Displaying notes found in: .note.gnu.property
+  Owner                 Data size	Description
+  GNU                  0x0000000c	NT_GNU_PROPERTY_TYPE_0
+      Properties: x86 feature: SHSTK

binutils/testsuite/binutils-all/i386/shstk.s

@@ -0,0 +1,27 @@
+	.section ".note.gnu.property", "a"
+.ifdef __64_bit__
+	.p2align 3
+.else
+	.p2align 2
+.endif
+	.long 1f - 0f		/* name length */
+	.long 5f - 2f		/* data length */
+	.long 5			/* note type */
+0:	.asciz "GNU"		/* vendor name */
+1:
+.ifdef __64_bit__
+	.p2align 3
+.else
+	.p2align 2
+.endif
+2:	.long 0xc0000002	/* pr_type.  */
+	.long 4f - 3f		/* pr_datasz.  */
+3:
+	.long 0x2
+4:
+.ifdef __64_bit__
+	.p2align 3
+.else
+	.p2align 2
+.endif
+5:

binutils/testsuite/binutils-all/x86-64/shstk-x32.d

@@ -0,0 +1,10 @@
+#source: shstk.s
+#PROG: objcopy
+#as: --x32
+#objcopy:
+#readelf: -n
+
+Displaying notes found in: .note.gnu.property
+  Owner                 Data size	Description
+  GNU                  0x0000000c	NT_GNU_PROPERTY_TYPE_0
+      Properties: x86 feature: SHSTK

binutils/testsuite/binutils-all/x86-64/shstk.d

@@ -0,0 +1,9 @@
+#PROG: objcopy
+#as: --64 -defsym __64_bit__=1
+#objcopy:
+#readelf: -n
+
+Displaying notes found in: .note.gnu.property
+  Owner                 Data size	Description
+  GNU                  0x00000010	NT_GNU_PROPERTY_TYPE_0
+      Properties: x86 feature: SHSTK

binutils/testsuite/binutils-all/x86-64/shstk.s

@@ -0,0 +1,27 @@
+	.section ".note.gnu.property", "a"
+.ifdef __64_bit__
+	.p2align 3
+.else
+	.p2align 2
+.endif
+	.long 1f - 0f		/* name length */
+	.long 5f - 2f		/* data length */
+	.long 5			/* note type */
+0:	.asciz "GNU"		/* vendor name */
+1:
+.ifdef __64_bit__
+	.p2align 3
+.else
+	.p2align 2
+.endif
+2:	.long 0xc0000002	/* pr_type.  */
+	.long 4f - 3f		/* pr_datasz.  */
+3:
+	.long 0x2
+4:
+.ifdef __64_bit__
+	.p2align 3
+.else
+	.p2align 2
+.endif
+5: