Update AArch64 ELF relocations to ABI 1.0

This mostly entails adding relocations, however there are a couple of
changes to existing relocations:

1. R_AARCH64_NONE is defined to be zero rather than 256

R_AARCH64_NONE has been defined to be zero for a long time elsewhere
e.g. binutils and glibc since the submission of the AArch64 port in
2012 so this is required for compatibility.

2. R_AARCH64_TLSDESC_ADR_PAGE renamed to R_AARCH64_TLSDESC_ADR_PAGE21

I don't think there is any way for relocation names to leak out of LLVM
so this should not break anything.

Tested with check-all with no regressions.