This function behaved differently on different architectures. Reimplement using mostly integer arithmatic.

Bug: b/136095012
Change-Id: I5df87e9400f2952386ebdf7b7b0866b0197145a8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33480Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>