Test Spike develop bug fixes in Ventus env CI

[liuqliang]

Summary

• advance the spike submodule from 44b121e9 to f891b873
• integrate the focused regext validity reset fix
• integrate the backward-compatible explicit GVM kernel start-PC API

Why

The simulator repository's develop pull requests do not trigger the Ventus integration workflow. ventus-env owns the recursive build and regression CI, so this companion PR lets the two simulator fixes run through the actual system-level checks.

This PR depends on THU-DSP-LAB/ventus-gpgpu-isa-simulator#50. The gitlink currently identifies the reviewed commit from the contributor fork; before this PR is merged, it should be updated to the corresponding commit reachable from the merged upstream simulator history.

Validation

• completed a full local simulator configure and make -j4 all
• passed focused processor-state coverage for reset and immediate/non-immediate transitions
• passed compile/link/run coverage for both GVM start APIs
• passed the two Ventus extra static test suites
• verified from a fresh shallow clone that git submodule update --init --depth=1 spike fetches and checks out f891b873e434529aaa3fe9fd99ae8ccb00f5cffa through the unchanged upstream submodule URL

The remaining build and regression coverage is intentionally delegated to this repository's CI.