Let clang decide ASan linker flags for the host.

Pass -fsanitize=address instead of manually specifying asan libraries
and other linker flags.

Note that we enable LOCAL_ALLOW_UNDEFINED_SYMBOLS by default for host
builds because ASAN only links symbols in the final executable, so
there will _always_ be undefined symbols in intermediate libraries.

Bug: 18208352
Change-Id: Ief55ab296e94974560eeb10507ec8d90f0025d5c