Log Message:
Update to 4.20

Changelog:
NSPR 4.20 contains the following changes:
- Added support for the RISC-V 64-bit target.
- Added support for aarch64 Windows.
- Added support for abstract socket addresses on Linux and Android.
- Fixed a FreeBSD build issue.
- ASAN poisoning functions on Windows no longer use the dllimport
  attribute.