[SystemZ] Add support for anyregcc calling convention

This adds back-end support for the anyregcc calling convention
for use with patchpoints.

Since all registers are considered call-saved with anyregcc
(except for 0 and 1 which may still be clobbered by PLT stubs
and the like), this required adding support for saving and
restoring vector registers in prologue/epilogue code for the
first time.  This is not used by any other calling convention.

llvm-svn: 326612