Fix bootstrap tools build on macOS after 02af91c52e71e8a0f47251e637c9687f35d45dd9

After changing the namespace.h header we need to provide _err on macOS, too.
Previously we used the system libc err*/warn*, but that does not provide
_err/_warn (which is used by other bootstrapped files from libc).
To fix this problem bootstrap err.c on macOS as well.

Fixes: 02af91c52 (Fix crossbuild bootstrap tools build with Clang 12)