Darwin, x86 - backport fix for mfentry test fails.

Darwin, x86 - backport fix for mfentry test fails.

This will fix the cases by UNSUPPORTING for any target that cannot
support mfentry (including Linux -fpic -m32).

gcc/

2019-05-30  Iain Sandoe  <iain@sandoe.co.uk>

	Backport from mainline.
	2019-05-23  Iain Sandoe  <iain@sandoe.co.uk>

	* config/i386/darwin.h: Reject -mfentry*.
	* doc/sourcebuild.texi: Document mfentry target support.

gcc/testsuite/

2019-05-30  Iain Sandoe  <iain@sandoe.co.uk>

	Backport from mainline.
	2019-05-23  Iain Sandoe  <iain@sandoe.co.uk>

       * lib/target-supports.exp (check_effective_target_mfentry): New.
       * gcc.target/i386/fentry-override.c: Require effective target mfentry.
       * gcc.target/i386/fentry.c: Likewise
       * gcc.target/i386/fentryname1.c: Likewise
       * gcc.target/i386/fentryname2.c: Likewise
       * gcc.target/i386/fentryname3.c: Likewise
       * gcc.target/i386/nop-mcount.c: Likewise
       * gcc.target/i386/pr82699-2.c: Likewise
       * gcc.target/i386/pr82699-4.c: Likewise
       * gcc.target/i386/pr82699-5.c: Likewise
       * gcc.target/i386/pr82699-6.c: Likewise
       * gcc.target/i386/returninst1.c: Likewise
       * gcc.target/i386/returninst2.c: Likewise
       * gcc.target/i386/returninst3.c : Likewise

From-SVN: r271767