[clang-cl] Use /arch: to set the base target CPU

The main effect of this change is that /arch:IA32 will use i386 as the
CPU, while clang-cl will continue to default to pentium4 (aka SSE2 plus
the usual other features).

/arch:AVX and /arch:AVX2 will also now enable the other features
available in sandybridge and haswell respectively, which is consistent
with MSDN.

llvm-svn: 241077