[asan] added a flag -mllvm asan-short-64bit-mapping-offset=1 (0 by default)
This flag makes asan use a small (<2G) offset for 64-bit asan shadow mapping.
On x86_64 this saves us a register, thus achieving ~2/3 of the
zero-base-offset's benefits in both performance and code size.

Thanks Jakub Jelinek for the idea.