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

@@ -296,13 +296,11 @@ 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

src/pkg/runtime/sys_netbsd_amd64.s

@@ -179,7 +179,6 @@ 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	$0xf1, 0xf1		// crash
@@ -253,11 +252,8 @@ TEXT runtime·sigaltstack(SB),7,$-8
 // 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