Support -masm= flag for x86 targets.

`clang -S -o - file.c -masm=att` will write assembly to stdout in at&t syntax
(the default), `-masm=intel` will instead output intel style asm.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208683 91177308-0d34-0410-b5e6-96231b3b80d8