[llgo] cmd/gllgo: handle/ignore more flags

Summary:
This diff is to support Debian packaging,
which sets various hardening-rleated flags
in CFLAGS. They don't make sense for Go,
so we just ignore them.

Reviewers: pcc

Subscribers: llvm-commits, axw

Differential Revision: http://reviews.llvm.org/D11288

llvm-svn: 242513