PPC: [wasm] Fix canonical signature id with no constant pool

Followup CL on porting f2aaa9fd

Change-Id: I2f36c60e214d2583622e9ad828045acd2b822ed3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5717298


Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#95132}