Fix strchr, strrchr implementation: convert c to char

(according to standard).

Discussed with:	andrew
Reviewed by:	emaste
Sponsored by:	DARPA, AFRL
Sponsored by:	HEIF5
Differential Revision:	https://reviews.freebsd.org/D8239

lib/libc/mips/string/strchr.S

@@ -44,7 +44,12 @@ __FBSDID("$FreeBSD$");
 	.abicalls
 #endif
 
+/*
+ * char *
+ * strchr(const char *s, int c);
+ */
 LEAF(strchr)
+	and		a1, a1, 0xff
 1:
 	lbu		a2, 0(a0)		# get a byte
 	PTR_ADDU	a0, a0, 1

lib/libc/mips/string/strrchr.S

@@ -44,8 +44,13 @@ __FBSDID("$FreeBSD$");
 	.abicalls
 #endif
 
+/*
+ * char *
+ * strrchr(const char *s, int c);
+ */
 LEAF(strrchr)
 	move		v0, zero		# default if not found
+	and		a1, a1, 0xff
 1:
 	lbu		a3, 0(a0)		# get a byte
 	PTR_ADDU	a0, a0, 1