[SystemZ] Fix applyFixup for 12-bit fixups

[SystemZ] Fix applyFixup for 12-bit fixups

Now that we have fixups that only fill parts of a byte, it turns
out we have to mask off the bits outside the fixup area when
applying them.  Failing to do so caused invalid object code to
be emitted for bprp with a negative 12-bit displacement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288374 91177308-0d34-0410-b5e6-96231b3b80d8