Fix Rotation Instruction Inference

[rnvannatta]

Hello,

This is a copy of https://github.com/imneme/pcg-cpp/pull/87 for your convenience to keep the C port of this library up to date, which is the version I am interested in.

In brief, it removes the need for the inline asm by adding a mask which exorcises some wacky C/C++ nasal demons and allows gcc and clang to infer a rot instruction.

Thanks, Richard