libc: core: implement memcmp and memmove

lib/libc/core/string/memcmp.c

@@ -0,0 +1,9 @@
+#include <stddef.h>
+
+int memcmp(const void *vl, const void *vr, size_t n)
+{
+	const unsigned char *l = vl, *r = vr;
+	for (; n && *l == *r; n--, l++, r++)
+		;
+	return n ? *l - *r : 0;
+}

lib/libc/core/string/memmove.c

@@ -0,0 +1,24 @@
+#include <stdint.h>
+#include <string.h>
+
+static void *memcpy_r(void *dest, const void *src, size_t sz)
+{
+	unsigned char *d = dest;
+	const unsigned char *s = src;
+
+	for (size_t i = 0; i < sz; i++) {
+		size_t b = sz - i - 1;
+		d[b] = s[b];
+	}
+
+	return dest;
+}
+
+void *memmove(void *dest, const void *src, size_t n)
+{
+	if (dest < src) {
+		return memcpy(dest, src, n);
+	} else {
+		return memcpy_r(dest, src, n);
+	}
+}