runtime: fix netbsd/386 stack pointer handling

When manipulating the stack pointer use the UESP register instead
of the ESP register, since the UESP register is the one that gets
restored from the machine context. Fixes broken tests on netbsd/386.

R=golang-dev, minux.ma, r, bsiegert
CC=golang-dev
https://golang.org/cl/6465054

src/pkg/runtime/signal_netbsd_386.c

@@ -29,7 +29,7 @@ runtime·dumpregs(McontextT *mc)
 	runtime·printf("edi     %x\n", mc->__gregs[REG_EDI]);
 	runtime·printf("esi     %x\n", mc->__gregs[REG_ESI]);
 	runtime·printf("ebp     %x\n", mc->__gregs[REG_EBP]);
-	runtime·printf("esp     %x\n", mc->__gregs[REG_ESP]);
+	runtime·printf("esp     %x\n", mc->__gregs[REG_UESP]);
 	runtime·printf("eip     %x\n", mc->__gregs[REG_EIP]);
 	runtime·printf("eflags  %x\n", mc->__gregs[REG_EFL]);
 	runtime·printf("cs      %x\n", mc->__gregs[REG_CS]);
@@ -47,7 +47,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 
 	if(sig == SIGPROF) {
 		runtime·sigprof((uint8*)mc->__gregs[REG_EIP],
-			(uint8*)mc->__gregs[REG_ESP], nil, gp);
+			(uint8*)mc->__gregs[REG_UESP], nil, gp);
 		return;
 	}
 
@@ -71,9 +71,9 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 		// (Otherwise the trace will end at runtime·sigpanic
 		// and we won't get to see who faulted.)
 		if(mc->__gregs[REG_EIP] != 0) {
-			sp = (uintptr*)mc->__gregs[REG_ESP];
+			sp = (uintptr*)mc->__gregs[REG_UESP];
 			*--sp = mc->__gregs[REG_EIP];
-			mc->__gregs[REG_ESP] = (uintptr)sp;
+			mc->__gregs[REG_UESP] = (uintptr)sp;
 		}
 		mc->__gregs[REG_EIP] = (uintptr)runtime·sigpanic;
 		return;
@@ -100,7 +100,7 @@ Throw:
 
 	if(runtime·gotraceback()){
 		runtime·traceback((void*)mc->__gregs[REG_EIP],
-			(void*)mc->__gregs[REG_ESP], 0, gp);
+			(void*)mc->__gregs[REG_UESP], 0, gp);
 		runtime·tracebackothers(gp);
 		runtime·dumpregs(mc);
 	}