Fix GCC 7 test failures.

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.

llvm-svn: 302488