[zhouguangyuan0718]

Compiler prerequisites #355 and #356 are merged. Based on 42dc37cc80160e792a97df75221af57263fc6a2f: package inputs have completed early IR preparation and LLVM pre-link O2 optimization.

Known blocker: the preceding Linux amd64/arm64 FullLTO CI run failed both cgo modes because runtime._cgo_clearenv has an indexed BSS definition in runtime and an initialized linkname definition in runtime/cgo. The current backend still rejects this as multiply defined; a minimal indexed-BSS/initialized-definition probe reproduces it. This PR is not ready to merge. Other integration cases in that run passed, but the subsequent standard-library step did not run.

Enable go build -lto=full end to end. Package bitcode is merged and optimized by LLVM, emitted as one GoObj, and consumed by the ordinary Go linker to produce executable and PIE binaries. The backend supports dynamic and static LLVM/plugin linkage and does not invoke external LLVM tools.

Content-addressed functions retain native hash classification. The compiler includes the compiling package in the existing inline-hash storage discriminator when creating LTO closures, including original closures and imported inline copies. LTO preserves those input names and rejects duplicate content-addressed identities; it no longer strips and regenerates their discriminators. GoObj strips the temporary discriminator before hashing and writing names. Identical machine code and relocations can merge across packages; different bodies stay distinct. No package-qualified output names or post-codegen name patching are needed.

The build driver selects bitcode compilation, includes LLVM/plugin identity in link cache keys, and leaves FIPS packages as ordinary objects. The linker rewrites archives only in a temporary directory, preserving assembly/native members and cached inputs. A package manifest restores imports, fingerprints, cgo directives, original symbol indices, logical compilation units and linkname origins over the single physical object. Mixed native objects, assembly references, initialization order and linkname checks remain supported.

LLVM handles native definition selection, reference remapping, ABI module flags, debug graphs and definition attachments. The Go adapter retains package/index provenance and Go-specific BSS requirements. Declared package names stay in native DWARF instead of being copied into the manifest. Linkname checks use original reference packages, allowing cross-package inlining without extra noinline restrictions. CPU/FMV preparation is performed by the compiler prerequisite; the LTO merger no longer constructs or runs a preparation pipeline, rejects unprepared compiler bitcode, and discards consumed CPU configuration rather than restoring main's copy. Saved input IR is written from the backend's existing parse in optimized/inputs, removing the extra Go-side parse. Compiler flag rejection remains covered by the compiler suite; the LTO suite tests build-driver restrictions.

Validation after rebasing and cleanup:

• 238/238 plugin CTests pass, including 46 FullLTO cases and amd64/arm64 GoObj hash, stack-map and debug-graph checks.
• Full integration suite passes locally on macOS arm64 (271.394s): cross-package optimization, cache reuse, content-addressed closures, assembly, mixed native objects, GC/stack growth, reflection, traceback, logical DWARF packages, linkname positive/negative cases, PIE and cgo callbacks with automatic/external linking.
• New amd64/arm64 compiler regressions verify distinct original/imported closure names and deterministic recompilation; they fail with the preceding compiler. FileCheck verifies unchanged input names, ABI0/FMV suffixes, content hashes and collision rejection in both input orders. A closure probe emits byte-identical ordinary objects before/after with both the native and non-LTO LLVM backends.
• Compiler slim-archive, pre-link preparation and incompatible-flag tests pass; the FullLTO CPU/FMV runtime fixture passes, including its GODEBUG child processes.
• Earlier validation: static LLVM/plugin linker builds and runs the runtime fixture with GOGC=1; compiler-bootstrap/non-cgo linker builds; focused archive, linker-version and dist tests pass.
• CI adds Linux amd64/arm64 FullLTO integration jobs and complete short suites for bytes, strings, strconv, sort, encoding/binary, encoding/json, hash/crc32 and unicode/utf8, without LTO-specific exclusions. Hosted results are the checks on the current PR head.

Scope: FullLTO at O2 for executable/PIE builds. ThinLTO, sanitizers, dynamic Go shared linking, separate compiler linkobj output, -N and custom optimization pipelines remain unsupported. FIPS inputs must retain ordinary object compilation.

Updates #341.