gas: sframe: add support for .cfi_b_key_frame

gas: sframe: add support for .cfi_b_key_frame

Gather the information from the DWARF FDE on whether frame's return
addresses are signed using the B key or A key.  Reflect the information in
the SFrame counterpart data structure, the SFrame FDE.

ChangeLog:

	* gas/gen-sframe.c (get_dw_fde_pauth_b_key_p): New definition.
	(sframe_v1_set_func_info): Add new argument for pauth_key.
	(sframe_set_func_info): Likewise.
	(output_sframe_funcdesc): Likewise.
	* gas/gen-sframe.h (struct sframe_version_ops): Add new argument
	to the function pointer declaration.
	* gas/sframe-opt.c (sframe_convert_frag): Handle pauth_key.