Require REX prefix to encode R_X86_64_GOTTPOFF

[H.J.]

As this linker bug:

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

shows that structions 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.  I checked the fix into GNU binutils trunk and 2.25 branch:

https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=cf61b7473a8ee3b43c4f8f2776075ec8d81d7495

I updated x86-64 psABI hjl/x32 branch:

https://github.com/hjl-tools/x86-64-psABI/commit/e3fb68774ffa0318158822851732a1425165ccf0

to require REX prefix to encode instructions with  R_X86_64_GOTTPOFF relocation.

H.J.

[H.J.]

Please ignore this.  I should posit it to x32 psABI group.