Out-of-tree LLIR scheduler + amdgcnas plugins. Removes the in-tree LLIR
scheduler and amdgcnas post-assembly tool (now shipped as plugins in the
gfx950-gluon-tutorials repo) and keeps only the plugin-enabling hooks: the
LLVM_PASS_PLUGIN_KEEP_TARGET_MACHINE gate that preserves the TargetMachine for
out-of-tree LLVM pass plugins, and the TRITON_FORCE_MFMA_AGPR flag that forces
AGPR-form MFMA accumulators (sets amdgpu-mfma-vgpr-form=0 in llvm.cc; the paired
amdgpu-agpr-alloc=256 is a kernel llvm_fn_attrs compile option). Companion to
the gfx950-gluon-tutorials repo at tag gfx950-tutorial-v1.0.