Requiring SSE2 by default for i386

After https://github.com/freebsd/freebsd-ports/commit/b039f2e46b15c1ff5cbaf41e642993d982294da3
, SSE2 is now set by default by default for lang/rust on i386.  And by
extension, it's also required for at least 500 ports that depend on
Rust.  So Pentium III users will now need to build their own packages
with Poudriere.  Given that, I think we might as well set SSE2 by
default for every other port, too.  The following ports currently
don't enable it by default for i386:

x11/babl
textproc/ugrep
misc/libpostal
graphics/fraqtive

Shall we do it?

A separate question is whether to update MACHINE_CPU to include sse2.
But that would affect base as well as ports.  Best to do things one at
a time.