Fix crossbuild bootstrap tools build with Clang 12

Clang 12 no longer allows re-defining a weak symbol as non-weak. This
happed here because we compile err.c with _err defined to err. To fix
this, use the same approach as the libc namespace.h

tools/build/libc-bootstrap/namespace.h

@@ -45,7 +45,9 @@
 #define _writev(a, b, c)	writev(a, b, c)
 #define _fsync(a)	fsync(a)
 #define	_getprogname()	getprogname()
-#define	_err(...)	err(__VA_ARGS__)
+/* These two need to be renamed to build libc/gen/err.c */
+#define	err	_err
+#define	warn	_warn
 
 #define _pthread_mutex_unlock	pthread_mutex_unlock
 #define _pthread_mutex_lock	pthread_mutex_lock

tools/build/libc-bootstrap/un-namespace.h

@@ -36,5 +36,8 @@
  * $FreeBSD$
  */
 #pragma once
-/* This can be empty when building the FreeBSD compatible bootstrap files */
+
+/* Undo the changes made by namespace.h */
+#undef	err
+#undef	warn