[ELF] Add -nopie alias for -no-pie (PR36423)

In r324043, --nopie was renamed to --no-pie to presumably fix a typo.

As it turns out, "nopie" wasn't a typo but the spelling used by
OpenBSD's binutils ld. Gold on the other hand spells the flag "no-pie".
(Vanilla binutils doesn't have a flag like this at all.)

Since they do the same thing, let's support both spellings.

llvm-svn: 325679