[PATCH][GCC][AARCH64] Add effective-target check to b key execution tests

[PATCH][GCC][AARCH64] Add effective-target check to b key execution tests

gcc/testsuite
2019-08-09  Sam Tebbs<sam.tebbs@arm.com>

	* lib/target-supports.exp
	(check_effective_target_arm_v8_4a_bkey_directive): New proc.
	* g++.target/aarch64/return_address_sign_b_exception.C,
	return_address_sign_ab_exception.C: Add dg-require-effective-target
	checks.

From-SVN: r274236