cmd/internal/obj/x86: implement -shared computation of TLS base

Somehow, terribly embarrassingly, I lost part of the "re-enable
-shared on amd64" patch when rebasing before it got submitted.
This restores it and also fixes the addend to be the necessary -4.

Change-Id: If71a09121d911a863bc07f1828ef76e3a54c1074
Reviewed-on: https://go-review.googlesource.com/6802Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/x86/asm6.go

@@ -3765,6 +3765,39 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 							ctxt.Andptr = ctxt.Andptr[1:]
 							asmand(ctxt, p, &pp.From, &p.To)
 
+						case obj.Hlinux:
+							if ctxt.Flag_shared == 0 {
+								log.Fatalf("unknown TLS base location for linux without -shared")
+							}
+							// Note that this is not generating the same insn as the other cases.
+							//     MOV TLS, R_to
+							// becomes
+							//     movq g@gottpoff(%rip), R_to
+							// which is encoded as
+							//     movq 0(%rip), R_to
+							// and a R_TLS_IE reloc. This all assumes the only tls variable we access
+							// is g, which we can't check here, but will when we assemble the second
+							// instruction.
+							ctxt.Rexflag = Pw | (regrex[p.To.Reg] & Rxr)
+
+							ctxt.Andptr[0] = 0x8B
+							ctxt.Andptr = ctxt.Andptr[1:]
+							ctxt.Andptr[0] = byte(0x05 | (reg[p.To.Reg] << 3))
+							ctxt.Andptr = ctxt.Andptr[1:]
+							r = obj.Addrel(ctxt.Cursym)
+							r.Off = int32(p.Pc + int64(-cap(ctxt.Andptr)+cap(ctxt.And[:])))
+							r.Type = obj.R_TLS_IE
+							r.Siz = 4
+							r.Add = -4
+							ctxt.Andptr[0] = 0x00
+							ctxt.Andptr = ctxt.Andptr[1:]
+							ctxt.Andptr[0] = 0x00
+							ctxt.Andptr = ctxt.Andptr[1:]
+							ctxt.Andptr[0] = 0x00
+							ctxt.Andptr = ctxt.Andptr[1:]
+							ctxt.Andptr[0] = 0x00
+							ctxt.Andptr = ctxt.Andptr[1:]
+
 						case obj.Hplan9:
 							if ctxt.Plan9privates == nil {
 								ctxt.Plan9privates = obj.Linklookup(ctxt, "_privates", 0)