[cmake] Move check for libproc to config-ix.cmake

As suggested by Nico in https://reviews.llvm.org/D96049, move check for libproc
from CMakeLists to config-ix.cmake

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D96713

Note: Also removes the CMAKE_REQUIRED_LIBRARIES which doesn't appear to
be necessary.