memmod: apply explicit build tags to _32 and _64 files

memmod: apply explicit build tags to _32 and _64 files

Since _32 and _64 aren't valid goarchs, they don't match _GOOS_GOARCH,
and so the existing tags wind up not being restricted to windows-only.
This fixes the problem by adding windows to the tags explicitly. We
could also fix it by calling the files _32_windows or _64_windows, but
that changes the convention with the other single-arch files.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>