This patch fixes the test failures and unexpected passes that occur
when testing against GCC 7. Specifically:
* don't mark __gcd as always inline because it's a recursive function. GCC diagnoses this.
* don't XFAIL the aligned allocation tests. GCC 7 supports them but not the -faligned-allocation option.
* Work around gcc.gnu.org/PR78489 in variants constructors.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302488 91177308-0d34-0410-b5e6-96231b3b80d8
