[lld] [ELF/AArch64] Fix R_AARCH64_ABS64 in Shared mode

This patch fixes the R_AARCH64_ABS64 relocation when used in shared mode,
where it requires a dynamic R_AARCH64_RELATIVE relocation. To correct set
the addend on the dynamic relocation (since it will be used by the dynamic
linker), a new TargetInfo specific hook was created (getDynRelativeAddend)
to get the correct addend based on relocation type.

The patch fixes the issues when creating shared library code against
{init,fini}_array, where it issues R_AARCH64_ABS64 relocation against
local symbols.

llvm-svn: 261651