[LLD][AArch64] Simplify relocations sharing same encoding [NFC]

The code to encode the result in relocateOne for the relocations:
R_AARCH64_LD64_GOT_LO12_NC
R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC
R_AARCH64_TLSDESC_LD64_LO12

is equivalent to that for R_AARCH64_LDST64_ABS_LO12_NC. This is described
in the ABI as "Set the LD/ST immediate field bits [11:3] of X. No overflow
check; check that X&7 =0.
    
Differential Revision: https://reviews.llvm.org/D46247

llvm-svn: 331452