Add a build script that invokes CMake to build the evmone library.
The script handles native, arm64, and mipsle targets, gracefully
skipping cross-compilation targets when the cross-compiler is not
available.
Move the script reference in ci.go from ./evmone/build.sh (inside
the submodule, not tracked) to ./build/evmone-build.sh (tracked
in the main repo).
