[PGO][AIX] Improve dummy var retention and allow -bcdtors:csect linking.

[PGO][AIX] Improve dummy var retention and allow -bcdtors:csect linking.

1) Use a static array of pointer to retain the dummy vars.
2) Associate liveness of the array with that of the runtime hook variable
   __llvm_profile_runtime.
3) Perform the runtime initialization through the runtime hook variable.
4) Preserve the runtime hook variable using the -u linker flag.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D136192