Reverse engineer the skinning matrix calculation and port to GOAL. This
is about 3x faster than the MIPS2c version.
As usual, there is a `*use-new-bones*` flag to go back to the old
version.
Fix for a bug in the compiler's `.div.vf` implementation (only happens
if src/dst are the same), and fix for a typo in the register allocator
that would sometimes cause it not to consider xmm8-xmm15.
