[libc] Remove test RPC opcodes from the exported header

This patch does the noisy work of removing the test opcodes from the
exported interface to an interface that is only visible in `libc`. The
benefit of this is that we both test the exported RPC registration more
directly, and we do not need to give this interface to users.

I have decided to export any opcode that is not a "core" libc feature as
having its MSB set in the opcode. We can think of these as non-libc
"extensions".

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D154848