cmd/dist: standardize GoALLC toolchain builds

[zhouguangyuan0718]

Summary

• keep src/make.bash as the only Go toolchain build entry; the canonical LLVM payload builder now lives in llvm-project
• make cmd/dist validate an installed payload, build/install the matching pass plugin, and invalidate plugin state from content rather than timestamps
• inherit the Darwin deployment target from the payload manifest when make.bash configures the plugin
• vendor the merged go-llvm Darwin static zstd support
• add Linux amd64 CI that consumes the versioned GoALLC LLVM Release rather than rebuilding LLVM
• validate the Release checksum, source revision, relocatable prefix, installed tools/headers/CMake package/libraries, then build Go and run plugin plus LLVM whitelist tests
• make LLVM whitelist tests select and validate one payload instead of mixing per-tool fallbacks
• build llvmtoolexec from the current checkout for the suite and select command-line-arguments in the wrapper, preserving recipe gcflags while guaranteeing LLVM plus default lowering
• document the payload, Release, ccache, make.bash, test-tool selection, verification, and upgrade contracts

Validation

• canonical llvm/utils/goallc/build-payload.bash completed a full Debug X86/AArch64 install locally with ccache enabled
• direct dynamic src/make.bash completed against that payload
• direct static make.bash completed against the same installed payload before the release split
• pass plugin CTest: 58/58 passed
• go test -count=1 cmd/dist cmd/llvmtoolexec
• real llc/plugin TestLLVMInitTaskOrder smoke test
• actionlint v1.7.7 and git diff --check
• representative LLVM runtime recipes passed with attached, all-pattern, and split-form gcflags
• representative runoutput and codegen tests passed while logging the configured installed payload and fresh wrapper

The Go CI downloads goallc-llvm23.1.0-v1-linux-amd64.tar.zst and its companion SHA-256 file. The Release build may still be publishing while this PR is reviewed; the workflow intentionally does not fall back to system LLVM or rebuild LLVM in the Go repository. Full whitelist feature failures were intentionally left out of scope for the test-mechanism cleanup.