Clean up header to reuse __riscv definition am: f26642ba30

Original change: https://android-review.googlesource.com/c/platform/external/boringssl/+/2461374

Change-Id: Ifcd161ced7de43780ffcc7a2eb4a35a91c92f3d2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>