Fix PR67974 for Darwin by adding an asm shim for AVX.

Fix PR67974 for Darwin by adding an asm shim for AVX.

The PR is about a set of tests that were not executed because of
missing asm support.

However, the assembler can handle the insns now, so the tests are
attempted (but with a missing asm shim they fail).

This adds the shim.

In principle (bugs notwithstanding) Darwin’s x86_64 ABI is supposed
to follow the sysv psABI so we can have essentially a copy of the
Linux one.

2018-12-24  Iain Sandoe  <iain@sandoe.co.uk>

	PR testsuite/67974
	* gcc.target/x86_64/abi/avx/asm-support-darwin.s: New.

From-SVN: r267395