cmd/ld: fix warnings on Plan 9

warning: src/cmd/ld/macho.c:595 sign-extended character constant
warning: src/cmd/ld/macho.c:595 sign-extended character constant
warning: src/cmd/ld/symtab.c:63 sign-extended character constant
warning: src/cmd/ld/symtab.c:63 sign-extended character constant

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/76580046

src/cmd/ld/macho.c

@@ -592,7 +592,7 @@ machosymtab(void)
 		} else {
 			p = s->extname;
 			while (*p++ != '\0') {
-				if(*p == '\xc2' && *(p+1) == '\xb7') {
+				if((uchar)*p == 0xc2 && (uchar)*(p+1) == 0xb7) {
 					adduint8(ctxt, symstr, '.');
 					p++;
 				} else {

src/cmd/ld/symtab.c

@@ -60,7 +60,7 @@ putelfstr(char *s)
 	if(p != nil) {
 		p = q = elfstrdat+off;
 		while (*q != '\0') {
-			if(*q == '\xc2' && *(q+1) == '\xb7') {
+			if((uchar)*q == 0xc2 && (uchar)*(q+1) == 0xb7) {
 				q += 2;
 				*p++ = '.';
 				elfstrsize--;