gas: sframe: testsuite: add testcase for .cfi_negate_ra_state

Add a new test to check that .cfi_negate_ra_state on aarch64 is handled
well (a non-empty SFrame section with valid SFrame FREs is generated).

ChangeLog:

	* testsuite/gas/cfi-sframe/cfi-sframe-aarch64-2.d: New test.
	* testsuite/gas/cfi-sframe/cfi-sframe-aarch64-2.s: Likewise.
	* testsuite/gas/cfi-sframe/cfi-sframe.exp: Adjust the list
	accordingly.