runtime: fix TLS handling for netbsd

Set the TLS base using the _lwp_setprivate() syscall, instead of via
sysarch(). NetBSD tracks the pointer passed to _lwp_setprivate() and
restores this value when restoring mcontext. If sysarch() is used
directly, restoring an mcontext trashes the FS/GS value, resulting
in a segfault when we next try to access the TLS.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6206062

src/pkg/runtime/sys_netbsd_386.s

@@ -12,14 +12,14 @@
 TEXT runtime·exit(SB),7,$-4
 	MOVL	$1, AX
 	INT	$0x80
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·exit1(SB),7,$-4
 	MOVL	$302, AX		// sys_threxit
 	INT	$0x80
 	JAE	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·write(SB),7,$-4
@@ -79,7 +79,7 @@ TEXT runtime·munmap(SB),7,$-4
 	MOVL	$73, AX			// sys_munmap
 	INT	$0x80
 	JAE	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·setitimer(SB),7,$-4
@@ -151,7 +151,7 @@ TEXT runtime·sigaction(SB),7,$24
 	MOVL	$340, AX		// sys___sigaction_sigtramp
 	INT	$0x80
 	JAE	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·sigtramp(SB),7,$44
@@ -193,7 +193,7 @@ TEXT runtime·sigtramp(SB),7,$44
 	MOVL	AX, 4(SP)		// arg 1 - sigcontext
 	MOVL	$103, AX		// sys_sigreturn
 	INT	$0x80
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 // int32 rfork_thread(int32 flags, void *stack, M *m, G *g, void (*fn)(void));
@@ -296,16 +296,14 @@ TEXT runtime·setldt(SB),7,$8
 
 TEXT runtime·settls(SB),7,$16
 	// adjust for ELF: wants to use -8(GS) and -4(GS) for g and m
-	MOVL	20(SP), CX
+	MOVL	base+0(FP), CX
 	ADDL	$8, CX
-	MOVL	CX, 0(CX)
 	MOVL	$0, 0(SP)		// syscall gap
-	MOVL	$16, 4(SP)		// X86_SET_GSBASE (x86/sysarch.h)
-	MOVL	CX, 8(SP)		// pointer to base
-	MOVL	$165, AX		// sys_sysarch
+	MOVL	CX, 4(SP)		// arg 1 - ptr
+	MOVL	$317, AX		// sys__lwp_setprivate
 	INT	$0x80
 	JCC	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·osyield(SB),7,$-4

src/pkg/runtime/sys_netbsd_amd64.s

@@ -83,13 +83,13 @@ TEXT runtime·exit(SB),7,$-8
 	MOVL	8(SP), DI		// arg 1 - exit status
 	MOVL	$1, AX			// sys_exit
 	SYSCALL
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·exit1(SB),7,$-8
 	MOVL	$302, AX		// sys_threxit
 	SYSCALL
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·write(SB),7,$-8
@@ -179,10 +179,9 @@ TEXT runtime·sigaction(SB),7,$-8
 	LEAQ	runtime·sigreturn_tramp(SB), R10
 	MOVQ	$3, R8			// arg 5 - version
 	MOVL	$340, AX		// sys___sigaction_sigtramp
-
 	SYSCALL
 	JCC	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·sigtramp(SB),7,$64
@@ -238,7 +237,7 @@ TEXT runtime·munmap(SB),7,$0
 	MOVL	$73, AX			// sys_munmap
 	SYSCALL
 	JCC	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·sigaltstack(SB),7,$-8
@@ -247,20 +246,17 @@ TEXT runtime·sigaltstack(SB),7,$-8
 	MOVQ	$281, AX		// sys___sigaltstack14
 	SYSCALL
 	JCC	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 // set tls base to DI
 TEXT runtime·settls(SB),7,$8
 	// adjust for ELF: wants to use -16(FS) and -8(FS) for g and m
-	ADDQ	$16, DI
-	MOVQ	DI, 0(SP)
-	MOVQ	SP, SI
-	MOVQ	$17, DI			// X86_64_SET_FSBASE (x86/sysarch.h)
-	MOVQ	$165, AX		// sys_sysarch
+	ADDQ	$16, DI			// arg 1 - ptr
+	MOVQ	$317, AX		// sys__lwp_setprivate
 	SYSCALL
 	JCC	2(PC)
-	MOVL	$0xf1, 0xf1  // crash
+	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 TEXT runtime·sysctl(SB),7,$0