Fix the AArch64 small object class lookup.
This was generating a relocation that didn't do the right thing and
didn't raise linker errors.  Now it is using GOT-relative addressing.
In combination with the last two commits, this now makes all of the
objc_msgSend tests pass on AArch64.
Fixes
ldp \receiver, x8, [sp], #(ARGUMENT_SPILL_SIZE + 16)
br x9
6:
- adr x10, SmallObjectClasses
+ adrp x10, :got:SmallObjectClasses
+ ldr x10, [x10, :got_lo12:SmallObjectClasses]
ldr x9, [x10, x9, lsl #3]
b 1b
.cfi_endproc
