Fix checks for buffer overflow when growing the buffer.

During relocation we may emit chunks of data bigger than the biggest
instruction.
Fix the return status of the compinfo tool.
Avoid preserving unused callee-saved registers.

When debug code is emitted, all callee-saved registers are saved and verified
not to be clobbered.
Rework the MacroAssembler::MemZero() routines.

The main point is to not use the loop instruction, which appears to be very
slow.
Use registers to store pointers to the beginning and end of the string.