[AIX][PowerPC][PGO] Generate .ref for some PGO sections

For PGO on AIX, when we switch to the linux-style PGO variable access
(via _start and _stop labels), we need the compiler to generate a .ref
assembly for each of the three csects:

 -   __llvm_prf_data[RW]
 -   __llvm_prf_names[RO]
 -   __llvm_prf_vnds[RW]

We insert the .ref inside the __llvm_prf_cnts[RW] csect so that if it's
live then the 3 csects are live.

For example, for a testcase with at least one function definition, when
compiled with -fprofile-generate we should generate:

        .csect __llvm_prf_cnts[RW],3
        .ref __llvm_prf_data[RW]   <<============ needs to be inserted
        .ref __llvm_prf_names[RO]  <<===========

the __llvm_prf_vnds is not always present, so we reference it only when
it's present.

Reviewed By: sfertile, daltenty

Differential Revision: https://reviews.llvm.org/D116607