gccrs: libproc_macro: Fix capacity update in tokenstream

The capacity was not updated on tokenstream grow. This commit also add a
new mechanism to prevent a tokenstream to grow with a zero delta capacity.

libgrust/ChangeLog:

	* libproc_macro/tokenstream.cc (TokenStream::grow): Add
	minimum growing capacity.
	(TokenStream::push): Change condition.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>