runtime: use netbsd signal ABI v2

Use version 2 of the NetBSD signal ABI - both version 2 and version 3
are supported by the kernel, with near identical behaviour. However,
the netbsd32 compat code does not allow version 3 to be used, which
prevents Go netbsd/386 binaries from running in compat mode on a
NetBSD amd64 kernel. Switch to version 2 of the ABI, which is the
same version currently used by NetBSD's libc.

R=minux.ma
CC=golang-dev
https://golang.org/cl/6476068

src/pkg/runtime/sys_netbsd_386.s

@@ -164,7 +164,7 @@ TEXT runtime·sigaction(SB),7,$24
 	MOVSL				// arg 3 - oact
 	LEAL	runtime·sigreturn_tramp(SB), AX
 	STOSL				// arg 4 - tramp
-	MOVL	$3, AX
+	MOVL	$2, AX
 	STOSL				// arg 5 - vers
 	MOVL	$340, AX		// sys___sigaction_sigtramp
 	INT	$0x80

src/pkg/runtime/sys_netbsd_amd64.s

@@ -183,7 +183,7 @@ TEXT runtime·sigaction(SB),7,$-8
 	MOVQ	24(SP), DX		// arg 3 - osa
 					// arg 4 - tramp
 	LEAQ	runtime·sigreturn_tramp(SB), R10
-	MOVQ	$3, R8			// arg 5 - version
+	MOVQ	$2, R8			// arg 5 - vers
 	MOVL	$340, AX		// sys___sigaction_sigtramp
 	SYSCALL
 	JCC	2(PC)