Enable SME2 Streaming SVE in ARM

[stevesuzuki-arm]

Enable SME2 Streaming SVE in ARM

This PR adds initial ARM SME2 streaming-mode support to Halide,
which allows us to compute with longer vector length SVE on targets with SME2.

A new sme_streaming(enable, var) scheduling directive provides the users
the option to control which loop is computed in streaming-mode.

The change introduces a new Target::SME2 feature and Target::streaming_vector_bits.
natural_vector_size() now depends on whether in streaming-mode or not,
because streaming_vector_bits may not be the same as vector_bits.

In Halide lowering, a new LowerSMEStreamingTasks pass is added,
which extracts the loop with streaming-mode as internal closure function
so that we can attach the LLVM function attributes to transit to/from streaming-mode.

• aarch64_pstate_sm_body to emit smstart/smstop transition
• NoInline to prevent streaming closure from inlined to non-streaming function

In CodeGen, target_vscale() depends on whether streaming-mode or not
and it varies even in a Module, although it is constant within Function boundary.
In streaming-mode, vector type code-gen and intrinsic selection are
performed based on streaming_vector_bits (streaming vscale).
In terms of coverage, it is almost the same as existing SVE2 code-gen
while SME2 specific instruction has not been enabled for now.

Additionally, the following changes are implemented:

• Auto-detect SME2 and streaming_vector_bits on host CPU
• Fall back from streaming SVE when vectorization factors are not feasible
• Gather/scatter in streaming mode is scalarized with warning
• Add runtime checks for streaming vscale mismatches with compile-time vscale

Breaking changes

• The signature of halide_error_vscale_invalid() is changed. Will consider to have separate API if necessary

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)

[stevesuzuki-arm]

This PR is ready for review. I will touch on this in dev meeting if I have a chance.