runtime: save context value in NetBSD sigtramp

On NetBSD a signal handler returns to the kernel by calling the
setcontext system call with the context passed to the signal handler.
The implementation of runtime·sigreturn_tramp for amd64, copied from the
NetBSD libc, expects that context address to be in r15.  That works in
the NetBSD libc because r15 is preserved across the call to the signal
handler.  It fails in the Go library because r15 is not preserved.
There are various ways to fix this; this one uses the simple approach,
essentially identical to the one in the NetBSD libc, of preserving r15
across the signal handler proper.

Looking at the code for 386 and arm suggests that they are OK.  However,
I have not actually tested them.

Update #14052.

Change-Id: I2b516b1d05fe5d3b8911e65ca761d621dc37fa1b
Reviewed-on: https://go-review.googlesource.com/18815
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_netbsd_amd64.s

@@ -245,11 +245,13 @@ TEXT runtime·sigfwd(SB),NOSPLIT,$0-32
 	CALL	AX
 	RET
 
-TEXT runtime·sigtramp(SB),NOSPLIT,$24
+TEXT runtime·sigtramp(SB),NOSPLIT,$32
 	MOVQ	DI, 0(SP)   // signum
 	MOVQ	SI, 8(SP)   // info
 	MOVQ	DX, 16(SP)  // ctx
+	MOVQ	R15, 24(SP) // for sigreturn
 	CALL	runtime·sigtrampgo(SB)
+	MOVQ	24(SP), R15
 	RET
 
 TEXT runtime·mmap(SB),NOSPLIT,$0