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
In most cases this simply builds the file from lib/libc for missing
functions (e.g. strlcpy on Linux etc.). In cases where this is not possible
I've added an implementation to tools/build/cross-build.
The fgetln.c/fgetwln.c/closefrom.c compatibility code was obtained from
https://gitlab.freedesktop.org/libbsd/libbsd, but I'm not sure it makes
sense to import it into to contrib just for these three bootstrap files.
Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D25978
