sframe.h: add support for .cfi_b_key_frame

ARM 8.3 provides five separate keys that can be used to authenticate
pointers. There are two key for executable (instruction) pointers. The
enum pointer_auth_key in gas/config/tc-aarch64.h currently holds two keys:
  enum pointer_auth_key {
    AARCH64_PAUTH_KEY_A,
    AARCH64_PAUTH_KEY_B
  };

Analogous to the above, in SFrame format V1, a bit is reserved in the SFrame
FDE to indicate which key is used for signing the frame's return addresses:
  - SFRAME_AARCH64_PAUTH_KEY_A has a value of 0
  - SFRAME_AARCH64_PAUTH_KEY_B has a value of 1

Note that the information in this bit will always be used along with the
mangled_ra_p bit, the latter indicates whether the return addresses are
mangled/contain PAC auth bits.

include/ChangeLog:

	* sframe.h (SFRAME_AARCH64_PAUTH_KEY_A): New definition.
	(SFRAME_AARCH64_PAUTH_KEY_B): Likewise.
	(SFRAME_V1_FUNC_INFO): Adjust to accommodate pauth_key.
	(SFRAME_V1_FUNC_PAUTH_KEY): New macro.
	(SFRAME_V1_FUNC_INFO_UPDATE_PAUTH_KEY): Likewise.

include/sframe.h

@@ -165,6 +165,10 @@ typedef struct sframe_header
 #define SFRAME_V1_HDR_SIZE(sframe_hdr)	\
   ((sizeof (sframe_header) + (sframe_hdr).sfh_auxhdr_len))
 
+/* Two possible keys for executable (instruction) pointers signing.  */
+#define SFRAME_AARCH64_PAUTH_KEY_A    0 /* Key A.  */
+#define SFRAME_AARCH64_PAUTH_KEY_B    1 /* Key B.  */
+
 typedef struct sframe_func_desc_entry
 {
   /* Function start address.  Encoded as a signed offset, relative to the
@@ -181,21 +185,30 @@ typedef struct sframe_func_desc_entry
      function.
      - 4-bits: Identify the FRE type used for the function.
      - 1-bit: Identify the FDE type of the function - mask or inc.
-     - 3-bits: Unused.
+     - 1-bit: PAC authorization A/B key (aarch64).
-     --------------------------------------------
+     - 2-bits: Unused.
-     |     Unused    |  FDE type |   FRE type   |
+     ------------------------------------------------------------------------
-     --------------------------------------------
+     |     Unused    |  PAC auth A/B key (aarch64) |  FDE type |   FRE type   |
-     8               5           4              0     */
+     |               |        Unused (amd64)       |           |              |
+     ------------------------------------------------------------------------
+     8               6                             5           4              0     */
   uint8_t sfde_func_info;
 } ATTRIBUTE_PACKED sframe_func_desc_entry;
 
 /* Macros to compose and decompose function info in FDE.  */
 
+/* Note: Set PAC auth key to SFRAME_AARCH64_PAUTH_KEY_A by default.  */
 #define SFRAME_V1_FUNC_INFO(fde_type, fre_enc_type) \
-  ((((fde_type) & 0x1) << 4) | ((fre_enc_type) & 0xf))
+  (((SFRAME_AARCH64_PAUTH_KEY_A & 0x1) << 5) | \
+   (((fde_type) & 0x1) << 4) | ((fre_enc_type) & 0xf))
 
 #define SFRAME_V1_FUNC_FRE_TYPE(data)	  ((data) & 0xf)
 #define SFRAME_V1_FUNC_FDE_TYPE(data)	  (((data) >> 4) & 0x1)
+#define SFRAME_V1_FUNC_PAUTH_KEY(data)	  (((data) >> 5) & 0x1)
+
+/* Set the pauth key as indicated.  */
+#define SFRAME_V1_FUNC_INFO_UPDATE_PAUTH_KEY(pauth_key, fde_info) \
+  ((((pauth_key) & 0x1) << 5) | ((fde_info) & 0xdf))
 
 /* Size of stack frame offsets in an SFrame Frame Row Entry.  A single
    SFrame FRE has all offsets of the same size.  Offset size may vary