clang-cl: Make all x86 CPU feature flags available, such as -msse3

Ideally, we would use the /arch cl.exe flag for this stuff.
Unfortunately, MSVC supports only 5 /arch flag values, which isn't
nearly enough to cover all the CPU features that LLVM cares about.  At
the very least, we need to know about SSE3 and SSE4.1 in addition to
SSE, SSE2, AVX, and AVX2.

In the future we should add the relevant /arch mappings in addition to
these gcc-style -m flags.