testsuite, jit: Allow for target-specific assembler scans.

testsuite, jit: Allow for target-specific assembler scans.

If we want to support multiple object formats and to allow for
scan-assembler tests, we need to make it possible to adjust the
tests on a per-target basis.

This adds similar mechamisms to jit-verify-assembler-output{,-not}
to those used for the general scan-assembler dg directives.

As an aside; it would, perhaps, be possible to integrate this more
with scanasm.exp (which would also give access to function body
scanning) but I did not attempt that for this patch.

After this, we can accept things like:
... { jit-verify-assembler-output-not "......" { target { ! *-*-darwin* } } } }
or
... { jit-verify-assembler-output "......" { target *-*-darwin* } } }

gcc/testsuite/ChangeLog:

	* jit.dg/jit.exp: Accept target clauses in jit-verify-assembler
	handling.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>