[OpenMP][AIX]Add assembly file containing microtasking routines and u…

…nnamed common block definitions (llvm#81770)

This patch adds assembly file `z_AIX_asm.S` that contains the 32- and
64-bit XCOFF version of microtasking routines and unnamed common block
definitions. This code has been run through the libomp LIT tests and a
user package successfully.

(cherry picked from commit 94100bc)