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.