AArch64: implement relocations for global access

AArch64: implement relocations for global access

The large memory model (default and main viable for JIT) emits
addresses in need of relocation as
    movz x0, #:abs_g3:somewhere
    movk x0, #:abs_g2_nc:somewhere
    movk x0, #:abs_g1_nc:somewhere
    movk x0, #:abs_g0_nc:somewhere

To support this we must implement those four relocations in the
dynamic loader.

This allows (for example) the test-global.ll MCJIT test to pass on
AArch64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181132 91177308-0d34-0410-b5e6-96231b3b80d8