log() was used instead of log2().

Also replace pow(2.0f, x) with the equivalent exp2(x), and remove
overloaded functions from Math.hpp which are guaranteed by C++11.

Bug: b/138944025
Change-Id: I9e87ee9b2afd8791a40dd3e73191e1ed0740acf5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34668
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>