broadcom: Add VC5 NIR compiler.

broadcom: Add VC5 NIR compiler.

This is a pretty straightforward fork of VC4's NIR compiler to VC5.  The
condition codes, registers, and I/O have all changed, making the backend
hard to share, though their heritage is still recognizable.

v2: Move to src/broadcom/compiler to match intel's layout, rename more
    "vc5" to "v3d", rename QIR to VIR ("V3D IR") to avoid symbol conflicts
    with vc4, use new v3d_debug header, add compiler init/free functions,
    do texture swizzling in NIR to allow optimization.