aarch64: skip copy of RA state register into target context

aarch64: skip copy of RA state register into target context

The RA state register is local to a frame, so it should not be copied to
the target frame during the context installation.

This patch adds a new backend handler that check whether a register
needs to be skipped or not before its installation.

libgcc/ChangeLog:

	* config/aarch64/aarch64-unwind.h
	(MD_FRAME_LOCAL_REGISTER_P): new handler checking whether a register
	from the current context needs to be skipped before installation into
	the target context.
	(aarch64_frame_local_register): Likewise.
	* unwind-dw2.c (uw_install_context_1): use MD_FRAME_LOCAL_REGISTER_P.