Pthread device: limit constant buffer size to 1/4 of global mem size.
Pthread device: limit constant buffer size to 1/4 of global mem size.

This is required for conformance tests

Oblomov/pocl,rjodin/pocl,jrprice/pocl,pocl/pocl,franz/pocl,franz/pocl,jrprice/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl,franz/pocl,jrprice/pocl,jrprice/pocl,pocl/pocl,pocl/pocl,rjodin/pocl,pocl/pocl,Oblomov/pocl,franz/pocl,rjodin/pocl,rjodin/pocl,pocl/pocl,Oblomov/pocl