Add x32 psABI support

Require REX prefix to encode R_X86_64_GOTTPOFF

Instructions with R_X86_64_GOTTPOFF relocation must be encoded with REX
prefix even if it isn't required by destination register.  Otherwise linker
can't safely perform IE -> LE optimization.  See:

https://sourceware.org/bugzilla/show_bug.cgi?id=17482