asm: encode %r12 with a SIB byte.
asm: encode %r12 with a SIB byte.

Since %r12 has the same three least significant bits as %rsp,
encoding it in ModR/M results in CPU looking for a (nonexistent)
SIB.

(Reported and fixed by @ElectronicRU.)

pyos/re2jit-template,pyos/re2jit,pyos/re2jit,pyos/re2jit,pyos/re2jit-template,pyos/re2jit-template