[libunwind][AArch64] Disable ZA before resuming from unwinding (on Linux) (#165451)
This patch reimplements the SME ABI `__arm_za_disable` routine within
libunwind. This routine must be called before resuming from unwinding on
AArch64 platforms with SME support.
Before calling the routine, we need to check that SME is available. In
this patch, this is implemented for Linux-based platforms by checking
HWCAP2. It should be possible to implement this check for other
platforms as required.
This patch includes a test for this functionality. This test requires
SME, so on platforms without it, it will simply pass.