CUDA device code is not hosted
CUDA device code is not hosted

When building the cuda driver with LLVM 7, compilation of the device
functions may fail with the error:

/usr/include/stdint.h:26:10: fatal error: 'bits/libc-header-start.h' file not found

Adding `-ffreestanding` to the compilation flags for the cuda driver
avoids attempts to access the host include files.

Fixes #671.

Oblomov/pocl,franz/pocl,franz/pocl,pocl/pocl,pocl/pocl,pocl/pocl,franz/pocl,pocl/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl,pocl/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl