libnspr: only pass --{enable,disable}-thumb2 on ARM

Fixes http://autobuild.buildroot.net/results/dff3f851ee4c62067a0b231c683383ce8692b8b3



libnspr adds -marm to CFLAGS when passed --disable-thumb2, which is only
recognized by ARM cross compilers, so only use when building for ARM.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>