darwin, x86 - disable '-mfentry' for the port.

This can't work without special support because placing
the call directly after the function label breaks the
ABI requirement that call sites are 16byte aligned.

gcc/

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

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

gcc/testsuite/

2019-05-23  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/testsuite/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: r271580