kernel: Use compiler-builtin memset, memcpy, memmove, memcmp.

Since the kernel is built with -ffreestanding, and thus -fno-builtin,
the compiler ignores the function names and doesn't perform optimizations
(e.g. inlining for small values) that it otherwise could. So, here,
we add a string.h that supplements the default, and uses #define to
reinstate the builtins, and thus the optimizations.

Linux and FreeBSD at least apparently do the same. A quick compile
benchmark in a VM doesn't show much difference, maybe a slight
decrease in sys time.

headers/private/kernel/string.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2026, Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef KERNEL_STRING_H
+#define KERNEL_STRING_H
+
+
+#include_next <string.h>
+
+
+#define memset(DEST, V, LEN)	__builtin_memset((DEST), (V), (LEN))
+#define memcpy(DEST, SRC, LEN)	__builtin_memcpy((DEST), (SRC), (LEN))
+#define memmove(DEST, SRC, LEN)	__builtin_memmove((DEST), (SRC), (LEN))
+#define memcmp(B1, B2, LEN)		__builtin_memcmp((B1), (B2), (LEN))
+
+
+#endif	/* KERNEL_STRING_H */