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

(cherry picked from commit 02af91c52e71e8a0f47251e637c9687f35d45dd9)