Remove custom -DLINUX pre-compile definitions

Please, remove custom -DLINUX and any similar custom pre-compile definitions:
```
# fgrep -rsn "DLINUX"
Tools/MediaDriverTools/GenKrnBin/CMakeLists.txt:34:add_definitions(-DLINUX_)
Tools/MediaDriverTools/KernelBinToSource/CMakeLists.txt:25:add_definitions(-DLINUX_)
Tools/MediaDriverTools/KrnToHex_IGA/CMakeLists.txt:29:add_definitions(-DLINUX_)
media_driver/cmake/linux/media_compile_flags_linux.cmake:76: -DLINUX=1
media_driver/cmake/linux/media_compile_flags_linux.cmake:77: -DLINUX
# fgrep -rsn "D_LINUX"
# fgrep -rsn "D__LINUX"
#
```

This requires cmake-level change and code-level change. Instead, please, use standard `__linux__` macro mandated by Linux kernel: http://www.faqs.org/docs/Linux-HOWTO/GCC-HOWTO.html#INDEX.25.

We consider to add installation of cmrt header files (see #445) and there is a risk that sooner or later our custom define will end up in them. We need to prevent this.