This allows Subzero to release the global initializers once they've been
lowered.

This CL also modifies the global initializer types to ensure they are
trivially destructible -- therefore not requiring destructors to run.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4360
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1776473007 .