6l: Relocate CMOV* instructions

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

src/cmd/6l/asm.c

@@ -206,15 +206,15 @@ adddynrel(Sym *s, Reloc *r)
 	case 256 + R_X86_64_GOTPCREL:
 		if(targ->dynimpname == nil || targ->dynexport) {
 			// have symbol
-			// turn MOVQ of GOT entry into LEAQ of symbol itself
-			if(r->off < 2 || s->p[r->off-2] != 0x8b) {
-				diag("unexpected GOT_LOAD reloc for non-dynamic symbol %s", targ->name);
+			if(r->off >= 2 && s->p[r->off-2] == 0x8b) {
+				// turn MOVQ of GOT entry into LEAQ of symbol itself
+				s->p[r->off-2] = 0x8d;
+				r->type = D_PCREL;
+				r->add += 4;
 				return;
 			}
-			s->p[r->off-2] = 0x8d;
-			r->type = D_PCREL;
-			r->add += 4;
-			return;
+			// unknown instruction (CMOV* maybe), use GOT
+			targ->dynimpname = targ->name;
 		}
 		addgotsym(targ);
 		r->type = D_PCREL;