Set compiler argument '-march:SSE2' only if target is Windows

[dpino]

When building mozangle on Linux, there are many warning message like this printed on stdout:

warning: [email protected]: clang: warning: argument unused during compilation: '-arch:SSE2' [-Wunused-command-line-argument]

The reason is compiler argument -arch:SSE2 is only valid for MSVC.

Only add compiler argument -arch:SSE2 is target platform is Windows.