Accept not only --foo but also -foo.

GNU ld manual says that multi-letter long option can be prefixed with
either -- or -. Therefore, we should accept not only --subsystem but
also -subsystem, for example.

There is one exception. If an option starts with "o", it should only be
prefixed with -- to avoid ambiguity with -o<filename> option.

Differential Revision: https://reviews.llvm.org/D37825

llvm-svn: 313286