write_context: allow mutating the ostream used

write_context: allow mutating the ostream used

Allowing the mutation of the ostream, allows heavy reuse of the
write_context as this is the distinction in most places where
write_context is used.

Hence, fixup various users of write_context and use common objects where
applicable.

	* include/abg-writer.h (set_ostream): Declare new function.
	* src/abg-writer.cc (write_context::m_ostream): Make this data
	member be a pointer rather than a reference.
	(write_context::{write_context, get_ostream): Adjust.  member.
	(write_context::set_ostream): Define new member function.
	(set_ostream): Define new free-form function.
	* tools/abidw.cc (load_corpus_and_write_abixml)
	(load_kernel_corpus_group_and_write_abixml): Use the feature of
	mutating the ostream and reuse the write_context in most cases.

Signed-off-by: Matthias Maennich <maennich@google.com>