cmd/gc: fix undefined behaviour warning in subr.c

Fixes warning

/home/dfc/go/src/cmd/gc/subr.c:3469:8: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64' (aka 'long'); cast to an unsigned type to negate this value to itself

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/141220043

src/cmd/gc/subr.c

@@ -3466,7 +3466,7 @@ smagic(Magic *m)
 	p = m->w-1;
 	ad = m->sd;
 	if(m->sd < 0)
-		ad = -m->sd;
+		ad = -(uvlong)m->sd;
 
 	// bad denominators
 	if(ad == 0 || ad == 1 || ad == two31) {