cmd/dist, pkg/runtime: Plan 9, 64-bit: Get PID from TLS; remove use of `_tos'.

Using offsets from Tos is cumbersome and we've had problems
in the past. Since it's only being used to grab the PID, we'll just
get that from the default TLS instead.

R=rsc, rminnich, npe
CC=golang-dev
https://golang.org/cl/6543049

src/cmd/dist/buildruntime.c

@@ -145,6 +145,12 @@ static struct {
 		"#define	g(r) 0(r)\n"
 		"#define	m(r) 8(r)\n"
 	},
+	{"amd64", "plan9",
+		"#define	get_tls(r)\n"
+		"#define	g(r) 0(GS)\n"
+		"#define	m(r) 8(GS)\n"
+		"#define	procid(r) 16(GS)\n"
+	},
 	{"amd64", "",
 		"// The offsets 0 and 8 are known to:\n"
 		"//	../../cmd/6l/pass.c:/D_GS\n"

src/pkg/runtime/defs_plan9_amd64.h

 // nothing to see here
-#define tos_pid 64
 #define PAGESIZE 0x200000ULL

src/pkg/runtime/rt0_plan9_amd64.s

@@ -3,11 +3,9 @@
 // license that can be found in the LICENSE file.
 
 TEXT _rt0_amd64_plan9(SB),7, $0
-	MOVQ	AX, _tos(SB)
 	MOVQ	$_rt0_amd64(SB), AX
 	MOVQ	SP, DI
 	JMP	AX
 
 DATA runtime·isplan9(SB)/4, $1
 GLOBL runtime·isplan9(SB), $4
-GLOBL _tos(SB), $8

src/pkg/runtime/sys_plan9_amd64.s

@@ -105,9 +105,8 @@ TEXT runtime·rfork(SB),7,$0
 	MOVQ	DX, g(AX)
 	MOVQ	BX, m(AX)
 
-	// Initialize AX from _tos->pid
-	MOVQ	_tos(SB), AX
-	MOVQ	tos_pid(AX), AX
+	// Initialize AX from pid in TLS.
+	MOVQ	procid(AX), AX
 	MOVQ	AX, m_procid(BX)	// save pid as m->procid
 	
 	CALL	runtime·stackcheck(SB)	// smashes AX, CX