[Driver] Allow -e entry but reject -eentry (#72804)

This short option taking an argument is unfortunate.

* If a cc1-only option starts with `-e`, using it for driver will not be
  reported as an error (e.g. commit
  6cd9886c88d16d288c74846495d89f2fe84ff827).
* If another `-e` driver option is intended but a typo is made, the
  option will be recognized as a `-e`.

`gcc -export-dynamic` passes `-export-dynamic` to ld. It's not clear
whether some options behave this way.

It seems `-Wl,-eentry` and `-Wl,--entry=entry` are primarily used. There
may also be a few `gcc -e entry`, but `gcc -eentry` is extremely rare or
not used at all. Therefore, we probably should reject the Joined form of
`-e`.