Bug 1906736 - Make IMPORT_LIBRARY a full path. a=RyanVM

This makes IMPORT_LIBRARY a full path, to match the change to
SHARED_LIBRARY so that the IMPORT_LIBRARY target can be found in
js/src/build/Makefile.in. Fixes regression in js standalone build 'make
install'.

Initial patch by Philip Chimento.

Original Revision: https://phabricator.services.mozilla.com/D215992

Differential Revision: https://phabricator.services.mozilla.com/D216772