[SystemZ] Generate XC loop for memset 0 of variable length.

Benchmarking has shown that it is worthwhile to implement a variable length
memset of 0 with XC (exclusive or) like gcc does, instead of using a libcall.

This requires the use of the EXecute Relative Long (EXRL) instruction which
can now be done in a framework that can also be used with other target
instructions (not just XC).

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D103865