libc: core: implement strrchr

lib/libc/core/string/strrchr.c

@@ -0,0 +1,14 @@
+#include <string.h>
+
+char *strrchr(const char *str, int c)
+{
+	size_t len = strlen(str);
+
+	for (int i = len - 1; i > 0; i--) {
+		if (str[i] == c) {
+			return (char *)&str[i];
+		}
+	}
+
+	return NULL;
+}