Log Message:
Update to 4.9.6

Changelog:
NSPR 4.9.6 has the following bug fixes:
* Bug 782214: Auto detect Android API version for x86. Fixed by Makoto
  Kato.
* Bug 807883: Add the PL_SizeOfArenaPoolExcludingPool function. Fixed by
  Nicholas Nethercote.
* Bug 812085: Initialize Windows CRITICAL_SECTIONs without debug info and
  with nonzero spin count. Fixed by  Chris Peterson.
* Bug 827271: Add support for Aarch64 (64-Bit ARM). Fixed by Al Stone and
  Riku Voipio.
* Bug 841651: PR_SetThreadPriority doesn't do anything when using Bionic.
  Fixed by Gabriele Svelto.