gas: testsuite: add an aarch64 testsuite for SCFI

Similar to the x86_64 testcases, all .s files contain the corresponding
CFI directives.  This has been done to aid maintainence only: CFI
annotations in .s files help convey the expected EH Frame / SFrame data
in a format-oblivious way. All the tests are executed with --scfi
command line option.  Hence, the CFI annotations in the test .s files
are skipped altogether by the GAS for processing.

gas/testsuite/:
	* gas/scfi/aarch64/scfi-aarch64.exp: New file.
	* gas/scfi/aarch64/scfi-ldrp-1.d: New test.
	* gas/scfi/aarch64/scfi-ldrp-1.l: New test.
	* gas/scfi/aarch64/scfi-ldrp-1.s: New test.
	* gas/scfi/aarch64/scfi-ldrp-2.d: New test.
	* gas/scfi/aarch64/scfi-ldrp-2.l: New test.
	* gas/scfi/aarch64/scfi-ldrp-2.s: New test.
	* gas/scfi/aarch64/scfi-stp-1.d: New test.
	* gas/scfi/aarch64/scfi-stp-1.l: New test.
	* gas/scfi/aarch64/scfi-stp-1.s: New test.
	* gas/scfi/aarch64/scfi-strp-2.d: New test.
	* gas/scfi/aarch64/scfi-strp-2.l: New test.
	* gas/scfi/aarch64/scfi-strp-2.s: New test.