Mark parameter as [[maybe_unused]]

The width parameter is only used by some of the CPU-specific checks.
Add [[maybe_unused]] to fix the build for CPUs that do not vary the
implementation by width.

Fixes aosp_riscv64-userdebug build.

Test: builds
Change-Id: I0f69cf7cf4febc1ac16488633c2f5a997bdafc62