aarch64: Fix return_address_sign_ab_exception.C regression

Hi all,

this is to fix the regression of
g++.target/aarch64/return_address_sign_ab_exception.C that I
introduced with d8dadbc9.

'aarch_ra_sign_key' for aarch64 ended up being non defined in the opt
file and the function attribute "branch-protection=pac-ret+leaf+b-key"
stopped working as expected.

This patch moves the definition of 'aarch_ra_sign_key' to the opt
files for both Arm back-ends.

Regards

  Andera Corallo

gcc/ChangeLog:

	* config/aarch64/aarch64-protos.h (aarch_ra_sign_key): Remove
	declaration.
	* config/aarch64/aarch64.cc (aarch_ra_sign_key): Remove
	definition.
	* config/aarch64/aarch64.opt (aarch64_ra_sign_key): Rename
	to 'aarch_ra_sign_key'.
	* config/arm/aarch-common.cc (aarch_ra_sign_key): Remove
	declaration.
	* config/arm/arm-protos.h (aarch_ra_sign_key): Likewise.
	* config/arm/arm.cc (enum aarch_key_type): Remove definition.
	* config/arm/arm.opt: Define.