[AIX][Driver] Implement -mxcoff-build-id option

The -mxcoff-build-id=0xHEXSTRING option is an alternative to the
--build-id=0xHEXSTRING linker option that is not currently available in
the AIX linker.

If HEXSTRING is an odd number of hex digits then a '0' character is prepended.
The characters ':' and '-' are not allowed (unlike the GNU linker option).
The given build-id will be saved in the string table of the loader section.

A subsequent commit will teach the profile runtime to read and use the embedded id.

Reviewed By: daltenty, qiongsiwu1, stephenpeckham

Differential Revision: https://reviews.llvm.org/D146431