spec: add a spec function to join arguments

spec: add a spec function to join arguments

When passing `-o` flags to other options, the typical `-o foo` spelling
leaves a leading whitespace when replacing elsewhere. This ends up
creating flags spelled as `-some-option-with-arg= foo.ext` which doesn't
parse properly. When attempting to make a spec function to just remove
the leading whitespace, the argument splitting ends up masking the
whitespace. However, the intended extension *also* ends up being its own
argument. To perform the desired behavior, the arguments need to be
concatenated together.

gcc/:

	* gcc.cc (join_spec_func): Add a spec function to join all
	arguments.

Signed-off-by: Ben Boeckel <ben.boeckel@kitware.com>
Co-authored-by: Jason Merrill <jason@redhat.com>