Fix ident tests by Enabling ident output for X86 Darwin.

Fix ident tests by Enabling ident output for X86 Darwin.

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

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

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

From-SVN: r267372