[RVV] Enable SpaceMiT vendor IME (#24706)

## Summary

This PR adds an s8s8s32 mmt4d ukernel for SpaceMiT IME (`+xsmtvdot` /
`smt.vmadot`)
and wires the compiler to use it. The tile is fixed at 12x16x8 (3x4 grid
of 4x4x8 atoms). Under `+xsmtvdot` we
select that shape and auto-enable mmt4d; plain `+v` targets are
unchanged.

Also, runtime detection can't use the usual ISA hwprobe bits for this
vendor extension, so we set the feature bit from SpaceMiT X60
`mvendorid`/`marchid`. And adds lit coverage, mmt4d unit/benchmark
entries, and an
`riscv_64:ime:+v,+zvl256b,+xsmtvdot` e2e matmul variant. Validated on
Banana Pi (BPI-F3 / X60): cpuinfo, mmt4d_test, and the ukernel e2e path.



Fixes #24576

Co-authored-by: Coden  <github.com/coden-s>
Co-authored-by: Andrew <github.com/akuhlken>
Co-authored-by: Cursor <cursoragent@cursor.com>
Signed-off-by: Terence <terrencemahlatini@gmail.com>

---------

Signed-off-by: Terence <terrencemahlatini@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>