libnspr: fix 64-bit build

On some architectures (found on x86_64, but probably others too),
libnspr builds with -m32 unless it is configured for 64-bit build.  So
force 64-bit build on 64-bit architectures.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>