Make objfile::static_links an htab_up

Make objfile::static_links an htab_up

This changes objfile::static_links to be an htab_up, so that ~objfile
no longer has to explicitly destroy it.

Tested by the buildbot.

gdb/ChangeLog
2019-04-19  Tom Tromey  <tom@tromey.com>

	* symfile.c (reread_symbols): Update.
	* objfiles.c (objfile_register_static_link)
	(objfile_lookup_static_link): Update
	(~objfile) Don't delete static_links.
	* objfiles.h (struct objfile) <static_links>: Now an htab_up.