gcc-urlifier: handle option prefixes such as '-fno-'

Given e.g. this missppelled option (omitting the trailing 's'):
$ LANG=C ./xgcc -B. -fno-inline-small-function
xgcc: error: unrecognized command-line option '-fno-inline-small-function'; did you mean '-fno-inline-small-functions'?

we weren't providing a documentation URL for the suggestion.

The issue is the URLification code uses find_opt, which doesn't consider
the various '-fno-' prefixes.

This patch adds a way to find the pertinent prefix remapping and uses it
when determining URLs.
With this patch, the suggestion '-fno-inline-small-functions' now gets a
documentation link (to that of '-finline-small-functions').

gcc/ChangeLog:
	* gcc-urlifier.cc (gcc_urlifier::get_url_suffix_for_option):
	Handle prefix mappings before calling find_opt.
	(selftest::gcc_urlifier_cc_tests): Add example of urlifying a
	"-fno-"-prefixed command-line option.
	* opts-common.cc (get_option_prefix_remapping): New.
	* opts.h (get_option_prefix_remapping): New decl.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>