Re-enable libbcc as a loadable plugin.

Bug: http://b/32448087

There is no OS-specific suffix support for
LOCAL_ALLOW_UNDEFINED_SYMBOLS, so I specify this now for all host
builds, rather than just Linux (which is the only consumer of the plugin
today).

Test: Built successfully and was able to run the loadable plugin tests
from tests/run-lit-tests.sh successfully.

Change-Id: I1090ab569e76579707456c65071717f7c94d46f7