eclass/ruby-fakegem.eclass: support non-default bindir

eclass/ruby-fakegem.eclass: support non-default bindir

By default gems can provide binaries to be bin-wrapped in /usr/bin in a
directory called "bin" in the gem. This is only a default, and it is
possible for the gem to indicate that another directory contains the
binaries to be bin-wrapped using the gemspec bindir option.
dev-ruby/rspec-core and dev-ruby/bundler are gems where the
binaries are placed in an "exe" directory.

This change introduces RUBY_FAKEGEM_BINDIR, defaulting to "bin" for
backward compatibility, allowing this directory to be specified.