Before this commit, there used to be a workaround in the `func.func`/`gpu.func` op lowering when the bare-pointer calling convention was enabled. This workaround "patched up" the argument materializations for memref arguments. This can be done directly in the argument materialization functions (as the TODOs in the code base indicate).

These have been replaced with atomicrmw fadd

Define subtarget features for atomic fmin/fmax support.

The flat/global support is a real messe. We had float/double support at
the beginning in gfx6 and gfx7. gfx8 removed these. gfx10 reintroduced them.
gfx11 removed the f64 versions again.

gfx9 partially reintroduced them, in gfx90a and gfx940 but only for f64.

The `gpu.func` op lowering accounts for memref arguments/results (both
"normal" and bare-pointer supported), but the `gpu.return` op lowering
did not. The lowering produced invalid IR that did not verify.

This commit uses the same lowering strategy as for `func.return` in the
`gpu.return` lowering. (The C++ implementation is copied. We may want to
share some code between `func` and `gpu` lowerings in the future.)

Functions that have the `nvvm.kernel` attribute should have 0 results.

Follow-up to 05ba5c0648ae5e80d5afce270495bf3b1eef9af4. uint32_t is
preferred over const MCExpr * in the section stack uses because it
should only be evaluated once. Change the paramter type to match.