Trivial patch to add support for RISC-V. `__riscv__` is the deprecated
definition available on older toolchains.
Bug: None
Link: https://webrtc-review.googlesource.com/c/src/+/222481
[adelva: cherry-picked back with conflict in AUTHORS only]
Change-Id: I0a028de8aedfd18e69c3260f3b810e7502ae4b10