Add output file argument to generate-errcodes.pl

This is in preparation for building postgres with meson / ninja.

meson's 'capture' (redirecting stdout to a file) is a bit slower than programs
redirecting output themselves (mostly due to a python wrapper necessary for
windows). That doesn't matter for most things, but errcodes.h is a dependency
of nearly everything, making it a bit faster seem worthwhile.

Medium term it might also be worth avoiding writing errcodes.h if its contents
didn't actually change, to avoid unnecessary recompilations.

Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/5e216522-ba3c-f0e6-7f97-5276d0270029@enterprisedb.com
