Add byteswap/error/libintl/ar/search headers + tsearch family, mempcpy, rawmemchr

Portable ELF tooling (elfutils/libelf, needed by Mesa's radeonsi driver) relies
on several standard headers and functions Red Bear's relibc lacked:

- <byteswap.h>: bswap_16/32/64 (compiler builtins).
- <error.h>: GNU error()/error_at_line() (inline).
- <libintl.h>: gettext family as the identity map (catalog-less system; matches
  glibc's no-catalog fallback behaviour).
- <ar.h>: Unix archive struct/magic (header-only).
- <search.h> + src/header/search: POSIX tsearch/tfind/tdelete/twalk + GNU
  tdestroy, implemented as an unbalanced BST (conforming; POSIX mandates no
  balancing).
- string: mempcpy (memcpy returning end) and rawmemchr (unbounded memchr).

All complete implementations, no stubs. Additive (no ABI break), so existing
sysroot binaries are unaffected.

include/byteswap.h

@@ -0,0 +1,17 @@
+/* byteswap.h — byte-order swapping macros.
+ *
+ * Red Bear OS / relibc. Mirrors the glibc <byteswap.h> interface. The macros
+ * lower to the compiler byte-swap builtins, so they are constant-foldable and
+ * need no runtime symbol. Provided because portable ELF/object tooling
+ * (elfutils/libelf, etc.) uses bswap_16/32/64 directly.
+ */
+#ifndef _BYTESWAP_H
+#define _BYTESWAP_H 1
+
+#include <stdint.h>
+
+#define bswap_16(x) __builtin_bswap16((uint16_t)(x))
+#define bswap_32(x) __builtin_bswap32((uint32_t)(x))
+#define bswap_64(x) __builtin_bswap64((uint64_t)(x))
+
+#endif /* byteswap.h */