Disable lto optimisation for gcc builds
Disable lto optimisation for gcc builds

Using LTO has been observed to cause link time errors when using gcc - disabling
for non-clang (gcc) builds.

googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc