Default e_machine to EM_IAMCU for i?86-*-elfiamcu

This patch sets the default ELF output format of assembler and linker to
EM_IAMCU when binutils is configured to i?86-*-elfiamcu target.

gas/

	* configure.tgt (arch): Set to iamcu for i386-*-elfiamcu target.
	* config/tc-i386.c (i386_mach): Support iamcu.
	(i386_target_format): Likewise.

ld/

	* configure.tgt: Support i[3-7]86-*-elfiamcu target.

ld/testsuite/

	* ld-i386/i386.exp (iamcu_tests): Run iamcu-4.
	* ld-i386/iamcu-4.d: New file.

ld/ChangeLog

@@ -1,3 +1,7 @@
+2015-05-11  H.J. Lu  <hongjiu.lu@intel.com>
+
+	* configure.tgt: Support i[3-7]86-*-elfiamcu target.
+
 2015-05-11  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* Makefile.am (ALL_EMULATION_SOURCES): Add eelf_iamcu.c.

ld/configure.tgt

@@ -304,6 +304,8 @@ x86_64-*-netbsd*)	targ_emul=elf_x86_64
 				        sed -e 's/netbsd/netbsdelf/'`;;
 			esac ;;
 i[3-7]86-*-netware)	targ_emul=i386nw ;;
+i[3-7]86-*-elfiamcu)	targ_emul=elf_iamcu
+			targ_extra_emuls=elf_i386 ;;
 i[3-7]86-*-elf*)	targ_emul=elf_i386
 			targ_extra_emuls=elf_iamcu ;;
 x86_64-*-elf*)		targ_emul=elf_x86_64

ld/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2015-05-11  H.J. Lu  <hongjiu.lu@intel.com>
+
+	* ld-i386/i386.exp (iamcu_tests): Run iamcu-4.
+	* ld-i386/iamcu-4.d: New file.
+
 2015-05-11  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* ld-i386/abs-iamcu.d: New file.

ld/testsuite/ld-i386/i386.exp

@@ -248,6 +248,7 @@ proc iamcu_tests {} {
     run_dump_test "iamcu-1"
     run_dump_test "iamcu-2"
     run_dump_test "iamcu-3"
+    run_dump_test "iamcu-4"
 }
 
 iamcu_tests

ld/testsuite/ld-i386/iamcu-4.d

@@ -0,0 +1,16 @@
+#source: dummy.s
+#ld: tmpdir/startiamcu.o tmpdir/fooiamcu.o
+#readelf: -h
+#target: i?86-*-elfiamcu
+
+ELF Header:
+  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
+  Class:                             ELF32
+  Data:                              2's complement, little endian
+  Version:                           1 \(current\)
+  OS/ABI:                            UNIX - System V
+  ABI Version:                       0
+  Type:                              EXEC \(Executable file\)
+  Machine:                           Intel MCU
+  Version:                           0x1
+#pass