Log Message:
Update to 4.21

Changelog:
NSPR 4.21 contains the following changes:
- Use MAP_SHARED for read-only file mappings on MacOS and Android.
- Changed prbit.h to use builtin function on aarch64.
- Test program build fixes.
- Minor fixes to support unified builds.
- Update library copyright notices.
- Removed Gonk/B2G references.