[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

test/std/language.support/support.dynamic/nothrow_t.fail.cpp

@@ -11,7 +11,7 @@
 
 // Before GCC 6, aggregate initialization kicks in.
 // See https://stackoverflow.com/q/41799015/627587.
-// UNSUPPORTED: gcc-4, gcc-5
+// UNSUPPORTED: gcc-5
 
 // struct nothrow_t {
 //   explicit nothrow_t() = default;