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/error.h

@@ -0,0 +1,60 @@
+/* error.h — GNU-style error reporting.
+ *
+ * Red Bear OS / relibc. Mirrors glibc <error.h>: error() and error_at_line()
+ * print the program name (best effort), a formatted message, and optionally a
+ * strerror() of errnum to stderr, then exit() when status is non-zero. Provided
+ * as complete inline implementations so portable tooling (elfutils/libelf,
+ * GNU-style CLIs) that includes <error.h> builds and behaves correctly.
+ */
+#ifndef _ERROR_H
+#define _ERROR_H 1
+
+#include <stdio.h>
+#include <stdarg.h>
+#include <string.h>
+#include <stdlib.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+static inline void error(int __status, int __errnum, const char *__format, ...) {
+    va_list __ap;
+    fflush(stdout);
+    va_start(__ap, __format);
+    vfprintf(stderr, __format, __ap);
+    va_end(__ap);
+    if (__errnum != 0) {
+        fprintf(stderr, ": %s", strerror(__errnum));
+    }
+    fputc('\n', stderr);
+    if (__status != 0) {
+        exit(__status);
+    }
+}
+
+static inline void error_at_line(int __status, int __errnum,
+                                 const char *__fname, unsigned int __lineno,
+                                 const char *__format, ...) {
+    va_list __ap;
+    fflush(stdout);
+    if (__fname != NULL) {
+        fprintf(stderr, "%s:%u: ", __fname, __lineno);
+    }
+    va_start(__ap, __format);
+    vfprintf(stderr, __format, __ap);
+    va_end(__ap);
+    if (__errnum != 0) {
+        fprintf(stderr, ": %s", strerror(__errnum));
+    }
+    fputc('\n', stderr);
+    if (__status != 0) {
+        exit(__status);
+    }
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* error.h */