clang-cl: Implement the -arch flag

Summary:
This implements the -arch flag for both x86 and x86-64 by letting
them affect the default target features we pass to cc1.  -m machine
flags will override the features set by -arch.

Reviewers: hansw

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4519

llvm-svn: 213083