compiler: unify resolved LLVM target configuration

[cpunion]

Summary

• resolve one shared LLVM triple, CPU, feature set, and target ABI for the driver and SSA backend
• snapshot requested and effective target specs, using an explicit legacy-surrogate fallback only for unavailable external backends or incompatible Go-visible layouts
• forward TargetABI through ABIName when creating the LLVM TargetMachine and verify it through emitted RISC-V ELF flags
• keep ordinary-build cache identities stable while including non-default LLVM configuration in cache keys
• preserve TargetMachine, TargetData, module, and context ownership across normal and failure paths

Dependency

This branch uses the ABIName binding merged in cpunion/llvm#2 at b20c3fb9f902. The temporary module replacement can be removed after that binding is available from the upstream LLVM module.

Validation

• full related package tests: internal/xtool/llvm, internal/crosscompile, internal/cabi, ssa, internal/build
• focused coroutine workflow commands on Go 1.24.11
• race test for internal/coro
• RISC-V lp64 versus backend-default lp64d object emission
• target compatibility audit for AVR, RISC-V, RP2040, WASI, and Nintendo Switch
• diff check and focused vet

Focused CI note

During coroutine-slice integration this branch intentionally runs the dedicated Coroutine workflow only. SSA keeps all vet analyzers except its pre-existing copylocks findings; the full upstream workflow matrix will be restored before final integration.

[gemini-code-assist]

Code Review

This pull request introduces support for resolved LLVM target specifications (including target triple, CPU, features, and ABI) across the build, cross-compilation, and SSA packages. It refactors target resolution to use a new TargetSpec struct, allowing fine-grained LLVM configurations while preserving legacy cache identities for default GOOS/GOARCH builds. Additionally, it adds layout compatibility checks to ensure that custom target machines match the Go-visible LLVM object layout, and includes comprehensive tests validating these target configurations and fallbacks. No review comments were provided, so there is no feedback to address.

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