libgcc: aarch64: Add SME unwinder support

libgcc: aarch64: Add SME unwinder support

To support the ZA lazy save scheme, the PCS requires the unwinder to
reset the SME state to PSTATE.SM=0, PSTATE.ZA=0, TPIDR2_EL0=0 on entry
to an exception handler. We use the __arm_za_disable SME runtime call
unconditionally to achieve this.
https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#exceptions

The hidden alias is used to avoid a PLT and avoid inconsistent VPCS
marking (we don't rely on special PCS at the call site). In case of
static linking the SME runtime init code is linked in code that raises
exceptions.

libgcc/ChangeLog:

	* config/aarch64/__arm_za_disable.S: Add hidden alias.
	* config/aarch64/aarch64-unwind.h: Reset the SME state before
	EH return via the _Unwind_Frames_Extra hook.