sframe.h: add support for .cfi_negate_ra_state

Use the last remaining bit in the 'SFrame FRE info' word to store whether
the RA is signed/unsigned with PAC authorization code: this bit is named
as the "mangled RA" bit.  This bit is still unused for x86-64.

The behaviour of the mangled-RA info bit in SFrame format closely
follows the behaviour of DW_CFA_AARCH64_negate_ra_state in DWARF.  During
unwinding, whenever an SFrame FRE with non-zero "mangled RA" bit is
encountered, it means the upper bits of the return address contain Pointer
Authentication code.  The unwinder, hence, must use appropriate means to
restore LR correctly in such cases.

include/ChangeLog:

	* sframe.h (SFRAME_V1_FRE_INFO_UPDATE_MANGLED_RA_P): New macro.
	(SFRAME_V1_FRE_MANGLED_RA_P): Likewise.