[libclc] Update build instructions in readme (#111369)

The configure Python script was removed by
d6e0e6d255a7d54a3873b7a5d048eee00ef6bb6d /
https://reviews.llvm.org/D69966.

The readme was never updated with the cmake way to do it. I couldn't
find any dedicated buildbots for this so I'm making an educated guess.
This is what built locally for me.

GitOrigin-RevId: 64f7e1b697d834f620b069f2c3dc701ed8ff732a
1 file changed