runtime: use a fence instruction before rdtsc

runtime: use a fence instruction before rdtsc

    This implements the same choices made in the gc runtime, except that
    for 32-bit x86 we only use the fence instruction if the processor
    supports SSE2.

    The code here is hacked up for speed; the gc runtime uses straight
    assembler.

    Reviewed-on: https://go-review.googlesource.com/97715

From-SVN: r258336