www/firefox: fix build on i386

The included protobuf tries to figure out optimal memory alignment
for some variables, but on i386 our library is not really helpful
with that. Adjust the alignment for the problematic case, and disable
one static_assert() checking for alignment values (borrowed from
devel/protobuf port).

(cherry picked from commit 0e5839c5f4464fbce310362b973101753f73e4be)