nss: replace USE_NSS_64 with _LP64 builtin.

fixes build for 32bit when passing USE_64 (which is questionable)...

in pkgsrc we declare all mips64* platforms as 64bit, and use USE_64.
However, netbsd/mips64 is using a 32bit ABI, so it is akin to passing
USE_64=1 for 32bit.

perhaps not declaring it a 64bit platform is correct, but this package
is one of the only few using this logic, and it's unfeasible to have
correct logic for 32bit/64bit.

this package has considerably more logic for USE_64 than for USE_NSS_64,
so to avoid inadvertent damage to other platforms, retain the USE_64=1
logic.

feel free to object to this option in the discussion on tech-pkg.

(maya)