[ELF/AArch64] Add support for R_AARCH64_ADR_GOT_PAGE and R_AARCH64_LD64_GOT_LO12_NC.

With these relocations, it is now possible to build a simple "hello world"
program for AArch64 Debian.

Differential revision: http://reviews.llvm.org/D14917

llvm-svn: 253957