Using =A is not the right constraint for x86-64.
rdtsc is actually used as an example in the gcc documentation
and for x86-64 something like "=a"(tickl),"=d"(tickh) should be
used. Instead of worrying about this we can just use the builtin.

Change-Id: Ied720de0861ff9231a4a4cdf85b75945fe478056
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34108
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Jeff Muizelaar <jmuizelaar@mozilla.com>