Fix usage of proc_macro2::Punct.

Restructure stdarch-gen-arm to use `Group`s with `Delimiter`s rather
than ad-hoc `Punct`s.

`Punct` should only be used to represent specific characters, and never
for bracket-like characters. Recent versions of `Punct::new` check this
with an assertion.

Note that there doesn't appear to be a way to emit a line break for
formatting reasons — `Punct::new('\n', ...)` no longer works — so this
also removes all blank lines between functions in the generated files.

Author: jacobbramley