[libc++] Purge mentions of GCC 4 from the test suite

We don't support GCC 4 and older according to the documentation, so
we should pretend it doesn't exist.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@372787 91177308-0d34-0410-b5e6-96231b3b80d8
32 files changed