Use -std=c11 for C11 conform/ tests.

Now that GCC 4.7 or later is required to build glibc, this patch makes
the conformance tests use -std=c11 for C11 tests instead of -std=c1x
-D_ISOC11_SOURCE.

Tested for x86_64 and x86 (conform/ tests).

	* conform/GlibcConform.pm ($CFLAGS{"ISO11"}): Use -std=c11 instead
	of -std=c1x -D_ISOC11_SOURCE.