[AIX] Support init priority

Support reserved [0-100] and non-reserved[101-65535] Clang/GNU init
priority values on AIX.
This patch maps Clang/GNU values into priority values used in sinit/sterm
functions. User can play with values and be able to get init to occur
before or after XL init and vice versa.

Differential Revision: https://reviews.llvm.org/D91272