cmd/gc: fix undefined behaviour warnings in mparith3.c

Update #8527

Fixes two warnings:

src/cmd/gc/mparith3.c:255:10: runtime error: shift exponent 52 is too large for 32-bit type 'int'
src/cmd/gc/mparith3.c:254:14: runtime error: shift exponent 52 is too large for 32-bit type 'int'

LGTM=rsc
R=r, dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/134940044

src/cmd/gc/mparith3.c

@@ -251,8 +251,8 @@ mpgetfltN(Mpflt *a, int prec, int bias)
 		s = minexp - e;
 		if(s > prec+1)
 			s = prec+1;
-		if((v & ((1<<s)-1)) != 0)
-			v |= 1<<s;
+		if((v & ((1ULL<<s)-1)) != 0)
+			v |= 1ULL<<s;
 		v >>= s;
 		e = minexp;
 	}