Introduce LTO property for -fwhole-program-vtables cflag

-fwhole-program-vtables cflag requires LTO to be enabled. Some projects
add this flag unconditionally, breaking builds that turn off LTO.

Make this an LTO property so that build system will handle adding the
flag when LTO is on.

Test: build
Bug: 169004486
Change-Id: Ifce12c09e1d6b8c2807314c3bb982b076efcaaa4