gas: sframe: testsuite: add testcase for .cfi_b_key_frame

This is actually a composite test that checks SFrame unwind information
generation for both the .cfi_negate_ra_state and .cfi_b_key_frame
directives on aarch64.

ChangeLog:

	* testsuite/gas/cfi-sframe/cfi-sframe-aarch64-pac-ab-key-1.d:
	New test.
	* testsuite/gas/cfi-sframe/cfi-sframe-aarch64-pac-ab-key-1.s:
	Likewise.
	* testsuite/gas/cfi-sframe/cfi-sframe.exp: Run new test.