Merge "Add an enum for the riscv64 <sys/ucontext.h> register constants." am: 9118450303

Original change: https://android-review.googlesource.com/c/platform/bionic/+/2576815

Change-Id: I1f1c2685dc224110c71b9be15f72aef032787ce8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>