Compute caller save exclusion RegMasks once

By computing the RegMasks used to exclude caller saved registers only once, we can save some computation per C2 compilation. This saves about 27k instructions per compilation on x64, about 3% of the total compilation time for the stub code compiles that happen during startup. With 14 such compilations this saves around 380k instructions during bootstrap alone.

Proof-of-concept patch: http://cr.openjdk.java.net/~redestad/scratch/static_debug_masks.00/