zip_writer.cc: update powerof2 and use it unconditionally

Update the powerof2 macro to one that is integer overflow safe.
This definition is the same one used by bionic, but cleaned up using
clang_format.

Unconditionally use this powerof2 macro, to ensure consistency across
all the build environments.

Test: compiles and boots
Change-Id: I21bfaf2360d1f77caf96365e20490b1deadfdfe0