[AIX][PGO] Teach profile runtime to read build-id

On AIX, the build-id can be embedded in a binary using the -mxcoff-build-id
compiler option. When present, the build id is stored as an ascii string at the
beginning of the string table in the loader section of the XCOFF file.

Reviewed By: stephenpeckham, daltenty

Differential Revision: https://reviews.llvm.org/D146976