libnss: Enable for MIPS64 n32 platforms

After talking with upstream we concluded that MIPS64 n32 is treated as a
32-bit architecture by libnss because it has 32-bit pointers, so
USE_64=1 should not be set in that case.

Upstream bug:
   https://bugzilla.mozilla.org/show_bug.cgi?id=1010730

WRT:
   http://git.buildroot.net/buildroot/commit/?id=73e6ae0fe6a69f88183ae33401f2314088b5f799



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>