For Darwin, do not let -mcpu override the -arch option. <rdar://11059238>

On Darwin the architecture and the corresponding Mach-O slice is typically
specified with -arch.  If not, it defaults to the current host architecture.
Do not use -mcpu to override the -arch value.  This is only an issue when
people need to use specialized code for a non-default CPU (hopefully guarded
by run-time checks to detect the current processor).  The -mcpu option is
still used for the -target-cpu option to clang, but this patch causes it to
not be used to set the architecture in the target triple.

llvm-svn: 153197