llvm: expose target ABI name option

[cpunion]

Summary

• add ABIName to TargetMachineOptions and apply it before target-machine construction
• use the official target-machine options C API on LLVM 18+, with an equivalent LLVM 14-17 compatibility shim
• verify RISC-V lp64 versus lp64d through emitted ELF flags and floating-point calling convention

Motivation

LLGo target resolution carries an explicit target ABI. Applying it at target-machine construction avoids guessing LLVM backend defaults and is required for correct calling conventions and object metadata.

Validation

• full tests: default LLVM 19 and tagged LLVM 14, 18, 19, 20
• modern API path also exercised with local LLVM 22; PR CI covers real LLVM 21
• go vet, gofmt, clang-format, and diff check

[gemini-code-assist]

Code Review

This pull request adds support for specifying the target ABI name (ABIName) when creating a target machine, updating both the Go API and the underlying C++ bindings to handle this across different LLVM versions (including LLVM 18+ and legacy versions). Feedback suggests initializing the JIT boolean variable to false in TargetBindings.cpp to prevent issues with uninitialized variables, and using t.Skipf instead of t.Fatal in the RISC-V test to gracefully handle environments where the RISC-V backend is not installed.

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