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The linker avoids a GOT indirection by turning a MOV into
a LEA, but with x86-64 GCC has started emitting CMOV*
instructions which break the existing logic.

This will generate errors such as:

  unexpected GOT reloc for non-dynamic symbol luaO_nilobject_

The CMOV* instructions may be emitted with normal code like:

  if (o >= L->top) return cast(TValue *, luaO_nilobject);
  else return o;

Which gets compiled into (relocation offset at 1b):

  13: 48 3b 47 10             cmp    0x10(%rdi),%rax
  17: 48 0f 43 05 00 00 00    cmovae 0x0(%rip),%rax
  1e: 00

This change will allow the indirection through the GOT to
avoid the problem in those cases.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4071044