gas: testsuite: add a x86_64 testsuite for SCFI

The testsuite for SCFI contains target-specific tests.

As all the tests are executed with --scfi command line option, the CFI
annotations in the test .s files are skipped altogether by the GAS for
processing.  The CFI directives in the assembly files are added with the
intention to aid maintainence only: CFI annotations in .s files help
convey the expected EH Frame / SFrame data in a format-oblivious way.

Some testcases are used to highlight those asm constructs that the SCFI
machinery in GAS currently does not support:

  - Only AMD64 ABI is supported for now. Using --m32 with --scfi results
    in hard error.
    See scfi-unsupported-1.s.

  - Untraceable stack-pointer manipulation in function epilougue and prologue.
    See scfi-unsupported-2.s.

  - Using Dynamically realigned arguement pointer (drap) register.
    For SCFI, the CFA must be only REG_SP or REG_FP based.
    See scfi-unsupported-drap-1.s

Some testcases are used to highlight some diagnostics that the SCFI
machinery in GAS currently issues, with an intent to help user correct
inadvertent errors in their hand-written asm.  An error is issued in a
situation where GAS is not sure it will be able to synthesize valid CFI.

  - (#1) "Warning: SCFI: Asymetrical register restore"
  - (#2) "Error: SCFI: usage of REG_FP as scratch not supported"
  - (#3) "Error: SCFI: unsupported stack manipulation pattern"

In case of (#2) and (#3), SCFI generation is skipped for the respective
function.  Above is a subset of the warnings/errors implemented in the
code.

gas/testsuite/:
	* gas/scfi/README: New file.
	* gas/scfi/x86_64/scfi-add-1.d: New test.
	* gas/scfi/x86_64/scfi-add-1.s: New test.
	* gas/scfi/x86_64/scfi-add-2.d: New test.
	* gas/scfi/x86_64/scfi-add-2.s: New test.
	* gas/scfi/x86_64/scfi-bp-sp-1.d: New test.
	* gas/scfi/x86_64/scfi-bp-sp-1.s: New test.
	* gas/scfi/x86_64/scfi-callee-saved-1.d: New test.
	* gas/scfi/x86_64/scfi-callee-saved-1.s: New test.
	* gas/scfi/x86_64/scfi-callee-saved-2.d: New test.
	* gas/scfi/x86_64/scfi-callee-saved-2.s: New test.
	* gas/scfi/x86_64/scfi-callee-saved-3.d: New test.
	* gas/scfi/x86_64/scfi-callee-saved-3.s: New test.
	* gas/scfi/x86_64/scfi-callee-saved-4.d: New test.
	* gas/scfi/x86_64/scfi-callee-saved-4.s: New test.
	* gas/scfi/x86_64/scfi-cfg-1.d: New test.
	* gas/scfi/x86_64/scfi-cfg-1.s: New test.
	* gas/scfi/x86_64/scfi-cfg-2.d: New test.
	* gas/scfi/x86_64/scfi-cfg-2.s: New test.
	* gas/scfi/x86_64/scfi-cofi-1.d: New test.
	* gas/scfi/x86_64/scfi-cofi-1.s: New test.
	* gas/scfi/x86_64/scfi-diag-1.l: New test.
	* gas/scfi/x86_64/scfi-diag-1.s: New test.
	* gas/scfi/x86_64/scfi-diag-2.l: New test.
	* gas/scfi/x86_64/scfi-diag-2.s: New test.
	* gas/scfi/x86_64/scfi-dyn-stack-1.d: New test.
	* gas/scfi/x86_64/scfi-dyn-stack-1.s: New test.
	* gas/scfi/x86_64/scfi-fp-diag-2.l: New test.
	* gas/scfi/x86_64/scfi-fp-diag-2.s: New test.
	* gas/scfi/x86_64/scfi-ignore-1.d: New test.
	* gas/scfi/x86_64/scfi-ignore-1.s: New test.
	* gas/scfi/x86_64/scfi-indirect-mov-1.d: New test.
	* gas/scfi/x86_64/scfi-indirect-mov-1.s: New test.
	* gas/scfi/x86_64/scfi-indirect-mov-2.d: New test.
	* gas/scfi/x86_64/scfi-indirect-mov-2.s: New test.
	* gas/scfi/x86_64/scfi-indirect-mov-3.d: New test.
	* gas/scfi/x86_64/scfi-indirect-mov-3.s: New test.
	* gas/scfi/x86_64/scfi-lea-1.d: New test.
	* gas/scfi/x86_64/scfi-lea-1.s: New test.
	* gas/scfi/x86_64/scfi-leave-1.d: New test.
	* gas/scfi/x86_64/scfi-leave-1.s: New test.
	* gas/scfi/x86_64/scfi-pushq-1.d: New test.
	* gas/scfi/x86_64/scfi-pushq-1.s: New test.
	* gas/scfi/x86_64/scfi-selfalign-func-1.d: New test.
	* gas/scfi/x86_64/scfi-selfalign-func-1.s: New test.
	* gas/scfi/x86_64/scfi-simple-1.d: New test.
	* gas/scfi/x86_64/scfi-simple-1.s: New test.
	* gas/scfi/x86_64/scfi-simple-2.d: New test.
	* gas/scfi/x86_64/scfi-simple-2.s: New test.
	* gas/scfi/x86_64/scfi-sub-1.d: New test.
	* gas/scfi/x86_64/scfi-sub-1.s: New test.
	* gas/scfi/x86_64/scfi-sub-2.d: New test.
	* gas/scfi/x86_64/scfi-sub-2.s: New test.
	* gas/scfi/x86_64/scfi-unsupported-1.l: New test.
	* gas/scfi/x86_64/scfi-unsupported-1.s: New test.
	* gas/scfi/x86_64/scfi-unsupported-2.l: New test.
	* gas/scfi/x86_64/scfi-unsupported-2.s: New test.
	* gas/scfi/x86_64/scfi-unsupported-cfg-1.l: New test.
	* gas/scfi/x86_64/scfi-unsupported-cfg-1.s: New test.
	* gas/scfi/x86_64/scfi-unsupported-drap-1.l: New test.
	* gas/scfi/x86_64/scfi-unsupported-drap-1.s: New test.
	* gas/scfi/x86_64/scfi-x86-64.exp: New test.