AMD_DEBUG Can Now Be Used In Place Of R600_DEBUG For RadeonSI Options

When setting various debug options for the RadeonSI Gallium3D driver -- like enabling its NIR back-end among many other options -- that has traditionally been done through the R600_DEBUG= environment variable. But that variable name makes little sense these days since RadeonSI doesn't even support the now-vintage R600 GPUs. Thankfully, AMD_DEBUG= is now a supported alternative...