go-llvm: select custom LLVM by orthogonal build tags

[zhouguangyuan0718]

Summary

• make the binding exclusively target GoALLC's customized LLVM; remove system LLVM, byollvm, and LLVM 14-22 compatibility paths
• keep LLVM API version (llvm23) independent from link mode (dynamicllvm or staticllvm)
• use ${SRCDIR}/llvm as the only payload path, supplied as an ignored symlink by the caller
• replace generated cgo files with small source files selected by build tags
• make static mode link the customized libLLVMGoALLC.a aggregate
• move the existing GoALLC extension APIs out of the Go vendor-only patch into this module
• update CI to exercise custom LLVM 23 in both modes

Depends on goallc/llvm-project#2 for the static aggregate target.

Validation

• go test -tags="llvm23 dynamicllvm" ./...
• go test -tags="llvm23 staticllvm" ./...
• consumed by three-stage dynamic, static, and default-parameter Go bootstrap builds in cmd: select custom LLVM through dist parameters go#2

[zhouguangyuan0718]

Updated to the smaller static-link design requested in review:

• LLVM version and link mode remain orthogonal build-tag axes.
• staticllvm now links a binding-local ${SRCDIR}/libLLVMGoALLC.a.
• The binding no longer depends on an LLVM-side custom CMake target.
• CI assembles the archive from normal component archives with llvm-config and llvm-ar before testing static mode.

Local validation passed for both llvm23,dynamicllvm and llvm23,staticllvm.

[zhouguangyuan0718]

CI follow-up: removed the 51-minute per-PR LLVM source build. Automatic checks now run only the honest LLVM-independent source-format validation (current run: 6 seconds). Full llvm23,dynamicllvm and llvm23,staticllvm tests will be restored by downloading project-published prebuilt LLVM payloads rather than rebuilding LLVM in this repository.