AArch64: Ensure regcache is reset between tests

AArch64: Ensure regcache is reset between tests

A recent change made the AArch64 self tests resuse the saved regs
cache, rather than creating a new one.  Ensure it is reset to default
values between tests.

Do this by splitting the reset functionality from trad_frame_alloc_saved_regs
into a new function.

Fixes selftest on AArch64.

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_analyze_prologue_test): Reset saved regs.
	* trad-frame.c (trad_frame_reset_saved_regs): New function.
	(trad_frame_alloc_saved_regs): Call trad_frame_reset_saved_regs.
	* trad-frame.h (trad_frame_reset_saved_regs): New declaration.