libmach: fix warnings.

Fixes #1706.

R=adg, rsc
CC=golang-dev
https://golang.org/cl/4413051

src/cmd/5g/peep.c

@@ -1134,7 +1134,7 @@ copyu(Prog *p, Adr *v, Adr *s)
 		if(v->type == D_REG) {
 			if(v->reg <= REGEXT && v->reg > exregoffset)
 				return 2;
-			if(v->reg == REGARG)
+			if(v->reg == (uchar)REGARG)
 				return 2;
 		}
 		if(v->type == D_FREG)
@@ -1152,7 +1152,7 @@ copyu(Prog *p, Adr *v, Adr *s)
 
 	case ATEXT:	/* funny */
 		if(v->type == D_REG)
-			if(v->reg == REGARG)
+			if(v->reg == (uchar)REGARG)
 				return 3;
 		return 0;
 	}

src/cmd/5l/obj.c

@@ -317,7 +317,7 @@ zaddr(Biobuf *f, Adr *a, Sym *h[])
 	a->sym = h[c];
 	a->name = Bgetc(f);
 
-	if(a->reg < 0 || a->reg > NREG) {
+	if((schar)a->reg < 0 || a->reg > NREG) {
 		print("register out of range %d\n", a->reg);
 		Bputc(f, ALAST+1);
 		return;	/*  force real diagnostic */

src/libmach/obj.c

@@ -215,7 +215,7 @@ processprog(Prog *p, int doautos)
 {
 	if(p->kind == aNone)
 		return 1;
-	if(p->sym < 0 || p->sym >= NNAMES)
+	if((schar)p->sym < 0 || p->sym >= NNAMES)
 		return 0;
 	switch(p->kind)
 	{