IREE: use some clever CMake hacks to avoid linker flags

Create riscv_springbok.cmake file.This overrides the `add_executable`
function to add an explicit link against the springbok intrinsic library.
Doing so wires the dependencies through CMake and avoids the need for
passing along flags for executables from the build system and scheduling
them sequentially.

The file can also be used to connect springbok BSP with other projects.

Change-Id: I8830580464f17db12d4d0de0bec48bcb29c8ec47