www/qt5-webengine: Fix build on -CURRENT

clang on CURRENT invokes ld.lld, which does not search LOCALBASE.
GN host tools such as character_data_generator then fail with
"unable to find library -levent".

Add USES=localbase:ldflags and LIBRARY_PATH so the host linker
can find ports libraries without relying on GNU ld following the
chromium and qt6-webengine patterns.

Approved by: blanket