aarch64: fix return address access with pac [PR94891][PR94791]

aarch64: fix return address access with pac [PR94891][PR94791]

This is a big hammer fix for __builtin_return_address (PR target/94891)
returning signed addresses (sometimes, depending on wether lr happens
to be signed or not at the time of call which depends on optimizations),
and similarly -pg may pass signed return address to _mcount
(PR target/94791).

At the time of return address expansion we don't know if it's signed or
not so it is done unconditionally.

2020-07-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>

gcc/ChangeLog:

	PR target/94891
	PR target/94791
	* config/aarch64/aarch64-protos.h (aarch64_return_addr_rtx): Declare.
	* config/aarch64/aarch64.c (aarch64_return_addr_rtx): New.
	(aarch64_return_addr): Use aarch64_return_addr_rtx.
	* config/aarch64/aarch64.h (PROFILE_HOOK): Likewise.