[Darwin, X86] Backport fix for .ident tests.

[Darwin, X86] Backport fix for .ident tests.

The c-c++-common tests fail (or XPASS depending on which) on Darwin
because if doesn't emit a.ident marker. For X86 Darwin, this is a
trivial oversight; the assembler supports the directive. We can
therefore use the default target hook there.

2019-09-08  Iain Sandoe  <iain@sandoe.co.uk>

	Backport from mainline
	2018-12-23  Iain Sandoe  <iain@sandoe.co.uk>

	* config/i386/darwin.h (TARGET_ASM_OUTPUT_IDENT): New.

From-SVN: r275494