Fix PR libstdc++/64883 Darwin headers use always_inline so don't test that

Fix PR libstdc++/64883 Darwin headers use always_inline so don't test that

    Because darwin system headers use always_inline rather than
    __always_inline__ the libstdc++ test will fail, even if our headers only
    use the reserved form of the attribute. Don't test it on Darwin, and
    assume that testing on other targets will catch any accidental misuses
    in libstdc++ headers.

2018-12-06  Jonathan Wakely  <jwakely@redhat.com>
	    Iain Sandoe  <iain@sandoe.co.uk>

            PR libstdc++/64883
            * testsuite/17_intro/headers/c++1998/all_attributes.cc: Don't test
            always_inline on Darwin.
            * testsuite/17_intro/headers/c++2011/all_attributes.cc: Likewise.
            * testsuite/17_intro/headers/c++2014/all_attributes.cc: Likewise.
            * testsuite/17_intro/headers/c++2017/all_attributes.cc: Likewise.
            * testsuite/17_intro/headers/c++2020/all_attributes.cc: Likewise.

Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>
From-SVN: r266863