Revisit libnspr port for ARCv3

[kolerov]

There is a patch for adding support of ARCv3 to libnspr in Buildroot source tree: 456c146. I reverted (3a8ddd2) this patch it is incorrect and does not define valid constants both for HS5x and HS6x targets. Moreover, I am not sure that HS6x implementation itself is correct. This must be investigated.

There are other questions regarding libnspr:

1. What was the original motivation for adding this patch?
2. How can we verify this implementation?