[in-process] In-process linkage.

Switched on by default.
In-process compilation is switched off for now (due to https://github.com/RadeonOpenCompute/ROCm-OpenCL-Driver/issues/13).
Temporary env var is added (till in-process compilation is switched off):

export AMD_OCL_LINK_IN_PROCESS=0 to switch off. export AMD_OCL_LINK_IN_PROCESS=1 or unset AMD_OCL_LINK_IN_PROCESS to switch back on.
Testing done: ocl_conformance 1.2: api, atomics, basic, buffers, commonfns, compiler

ROCm / ROCm-OpenCL-Driver