[ELF][AArch64] Fix adrp to undefined weak reference.
In the ABI for the 64-bit Arm architecture the section on weak references
During linking, the symbol value of an undefined weak reference is:
- Zero if the relocation type is absolute
- The address of the place if the relocation type is pc-relative.
The relocations associated with an ADRP are relative so we should resolve
the undefined weak reference to the place instead of 0. This matches GNU
Differential Revision: https://reviews.llvm.org/D55599