[AArch64] Relax cross-section branches

Because the code layout is not known during compilation, the distance of
cross-section jumps is not knowable at compile-time. Because of this, we
should assume that any cross-sectional jumps are out of range. This
assumption is necessary for machine function splitting on AArch64, which
introduces cross-section branches in the middle of functions. The linker
relaxes out-of-range unconditional branches, but it clobbers X16 to do
so; it doesn't relax conditional branches, which must be manually
relaxed by the compiler.

Differential Revision: https://reviews.llvm.org/D145211