AsmPrinter: Suppress warnings on GCC from r238362, NFC

GCC seems to have some overzealous warnings about strict aliasing.
Rafael reports that this patch suppresses them on GCC 4.9, and I'm
hoping this will work for GCC 4.7 as well.  I'll watch [1] and iterate
if necessary.

[1]: http://bb.pgr.jp/builders/clang-3stage-x86_64-linux/builds/8597

llvm-svn: 238447