[AArch64] Switch magic numbers to library functions in fixup

Using masks and bounds that are magic numbers means that defects
in bounds checking and offset masking are subtle and hard to
detect. https://reviews.llvm.org/D152841 is an example of this
type of defect. Switching to clearly readable library functions
makes defects less obfuscated.

Differential Revision: https://reviews.llvm.org/D152843