cmd: select custom LLVM through dist parameters

[zhouguangyuan0718]

Summary

• add -llvm-dir, -llvm-version, and -llvm-link to cmd/dist bootstrap / make.bash
• resolve the payload from explicit -llvm-dir, then GOALLC_LLVM_DIR, then $GOROOT/llvm
• default to LLVM 23 and dynamic linking; validate the payload version and selected library
• atomically map the resolved payload to the binding's ${SRCDIR}/llvm path with an ignored symlink
• inject orthogonal llvm23,dynamicllvm or llvm23,staticllvm build tags without using ambient CGO_* variables or generated files
• vendor go-llvm: select custom LLVM by orthogonal build tags go-llvm#3 and update the project architecture/build document

Depends on goallc/go-llvm#3 and, for static mode, goallc/llvm-project#2.

Validation

• src/make.bash -a (default $GOROOT/llvm, LLVM 23, dynamic)
• src/make.bash -a -llvm-dir=$GOROOT/llvm -llvm-link=dynamic -llvm-version=23
• src/make.bash -a -llvm-dir=$GOROOT/llvm -llvm-link=static -llvm-version=23
• bin/go test cmd/dist
• bin/go test -tags="llvm23 dynamicllvm" cmd/compile/internal/ssa cmd/objview
• standalone binding dynamic and static test suites

The local Debug LLVM was built with the macOS 26 SDK while Go targets macOS 16, so the existing deployment-target linker warning remains.

[zhouguangyuan0718]

Updated to move static aggregation into cmd/dist:

• Dynamic mode still uses the ${SRCDIR}/llvm payload link.
• Static mode asks payload-local llvm-config for normal component archives and merges them with payload-local llvm-ar into the vendored binding directory.
• Non-platform static dependencies such as zstd are resolved as archives internally; no ambient CGO_*, LLVMROOT, generated Go source, or LLVM source changes are used.
• The aggregate is cached using LLVM version, component list, and input archive metadata.
• Vendored go-llvm is updated to PR cmd/compile: add LLVM IR-only toolexec pipeline #3 commit 223f6a971614.
• The superseded LLVM PR cmd: select custom LLVM through dist parameters #2 is closed with zero net source diff.

Validation: standalone go-llvm dynamic/static tests passed; full Go three-stage make.bash passed in both -llvm-link=dynamic and -llvm-link=static. The existing macOS 26 LLVM payload versus macOS 16 Go target warnings remain and should be removed when rebuilding the payload with an aligned deployment target.

[zhouguangyuan0718]

Follow-up after go-llvm PR #3 merged:

• Updated the dependency to merged go-llvm master commit 33267b8eb4a0 (v0.0.0-20260727072003-33267b8eb4a0).
• Removed GOALLC_LLVM_DIR, GOALLC_LLVM_VERSION, and GOALLC_LLVM_LINK as configuration inputs.
• LLVM payload, API version, and link mode now use only -llvm-dir, -llvm-version, and -llvm-link, with defaults $GOROOT/llvm, 23, and dynamic.

Validation: full three-stage builds passed in both dynamic and static modes while the removed environment variables were deliberately set to invalid values, confirming that only the command-line parameters are honored.