[SystemZ] Emit EXRL target instructions before text section is ended.

SystemZ adds the EXRL target instructions in the end of each file. This must
be done before debug info emission since that may end the text section, and
therefore this is now done in emitConstantPools() (instead of in
emitEndOfAsmFile).

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D109513