panfrost: Implement Midgard shader toolchain

panfrost: Implement Midgard shader toolchain

This patch implements the free Midgard shader toolchain: the assembler,
the disassembler, and the NIR-based compiler. The assembler is a
standalone inaccessible Python script for reference purposes. The
disassembler and the compiler are implemented in C, accessible via the
standalone `midgard_compiler` binary. Later patches will use these
interfaces from the driver for online compilation.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Emil Velikov <emil.velikov@collabora.com>