riscv64: define CPU context.

Also fix a few misleading comments for other architectures.

Test: lunch aosp_riscv64-userdebug && m dist
Co-authored-by: Ulya Trafimovich <skvadrik@google.com>

Change-Id: Iad9cb5559dea8b9463a2b130454044ae361c804e
10 files changed