syscall: use correct pointer in recvfrom/sendto.

linux/386 stack trace: use 32-bit hex.

Fixes #159.

R=r
https://golang.org/cl/154178

src/pkg/runtime/linux/386/signal.c

@@ -10,19 +10,19 @@
 void
 dumpregs(Sigcontext *r)
 {
-	printf("eax     %X\n", r->eax);
-	printf("ebx     %X\n", r->ebx);
-	printf("ecx     %X\n", r->ecx);
-	printf("edx     %X\n", r->edx);
-	printf("edi     %X\n", r->edi);
-	printf("esi     %X\n", r->esi);
-	printf("ebp     %X\n", r->ebp);
-	printf("esp     %X\n", r->esp);
-	printf("eip     %X\n", r->eip);
-	printf("eflags  %X\n", r->eflags);
-	printf("cs      %X\n", r->cs);
-	printf("fs      %X\n", r->fs);
-	printf("gs      %X\n", r->gs);
+	printf("eax     %x\n", r->eax);
+	printf("ebx     %x\n", r->ebx);
+	printf("ecx     %x\n", r->ecx);
+	printf("edx     %x\n", r->edx);
+	printf("edi     %x\n", r->edi);
+	printf("esi     %x\n", r->esi);
+	printf("ebp     %x\n", r->ebp);
+	printf("esp     %x\n", r->esp);
+	printf("eip     %x\n", r->eip);
+	printf("eflags  %x\n", r->eflags);
+	printf("cs      %x\n", r->cs);
+	printf("fs      %x\n", r->fs);
+	printf("gs      %x\n", r->gs);
 }
 
 /*

src/pkg/syscall/syscall_linux_386.go

@@ -125,7 +125,7 @@ func setsockopt(s int, level int, name int, val uintptr, vallen int) (errno int)
 func recvfrom(s int, p []byte, flags int, from *RawSockaddrAny, fromlen *_Socklen) (n int, errno int) {
 	var base uintptr;
 	if len(p) > 0 {
-		base = uintptr(unsafe.Pointer(&p))
+		base = uintptr(unsafe.Pointer(&p[0]))
 	}
 	n, errno = socketcall(_RECVFROM, uintptr(s), base, uintptr(len(p)), uintptr(flags), uintptr(unsafe.Pointer(from)), uintptr(unsafe.Pointer(fromlen)));
 	return;
@@ -134,7 +134,7 @@ func recvfrom(s int, p []byte, flags int, from *RawSockaddrAny, fromlen *_Sockle
 func sendto(s int, p []byte, flags int, to uintptr, addrlen _Socklen) (errno int) {
 	var base uintptr;
 	if len(p) > 0 {
-		base = uintptr(unsafe.Pointer(&p))
+		base = uintptr(unsafe.Pointer(&p[0]))
 	}
 	_, errno = socketcall(_SENDTO, uintptr(s), base, uintptr(len(p)), uintptr(flags), to, uintptr(addrlen));
 	return;