As egreg said,
kpsewhich -a texmf.cnf will show you the location of the file. However, there are other ways. As I said
locate texmf.cnf is maybe the simplest.
Or you can look in the
# replace upstream texmf.cnf with ours
rm -f "$pkgdir"/usr/share/texmf-dist/web2c/texmf.cnf
install -m644 "$srcdir"/texmf.cnf "$pkgdir"/etc/texmf/web2c/texmf.cnf
# since the location of texmf.cnf is hard-wired to be under /usr/share/texmf/web2c
# we make a symlink from /etc/texmf/web2c/texmf.cnf to the latter
ln -sf /etc/texmf/web2c/texmf.cnf "$pkgdir"/usr/share/texmf-dist/web2c/texmf.cnf
This shows that the file is installed in
/usr/share/texmf-dist/web2c/texmf.cnf with a symlink from
/etc/texmf/web2c/texmf.cnf. But don't edit
/usr/share/texmf-dist/web2c/texmf.cnf - as the file itself points out, your changes will be overwritten on update if you do.
locate not be available, install it using
pacman -S mlocate