[compiler-rt] Split cflags and link flags to avoid warnings

Summary:
The MSVC compiler is complaining about invalid flags. The CFLAGS are passed
to the linker (i.e. /Gy-, ...).

Reviewers: rnk

Subscribers: llvm-commits, kubabrecka, chrisha

Differential Revision: http://reviews.llvm.org/D21554

llvm-svn: 273265