[ORCJIT] Embed liborc_rt into the extension and add orc_rt selector

[cyx-6]

Motivation

The ORC runtime shipped as a separate liborc_rt*.a data file next to the extension .so and was resolved at startup via a filesystem glob (through the tvm_ffi_orcjit.DefaultOrcRuntimePath Python hook). That couples two artifacts, lets the runtime be lost/relocated by repackaging, and does a filesystem glob to find its own runtime on first session.

What this does

Bakes liborc_rt.a into the extension .so's .rodata via a generated .incbin stub (orc_rt_embed.S.in) and hands the bytes to ExecutorNativePlatform as a zero-copy MemoryBuffer. The wheel now ships one artifact, startup does no path lookup, and the runtime cannot be separated from the extension. The embedded start/end symbols are .hidden, so they add no interposition surface and don't affect the existing --exclude-libs symbol hiding.

This is Solution A of the embedded-orcrt design.

default_session()

Always uses the embedded runtime and is deliberately not user-configurable: a shared process-wide singleton with a hidden runtime knob would let whichever caller runs first silently pick the platform for everyone. The old DefaultOrcRuntimePath discovery hook is dropped.

User-created ExecutionSession — new orc_rt selector

Renamed orc_rt_path → orc_rt, now a 4-state selector:

orc_rt=	meaning
"auto" (default)	the embedded runtime
str / Path	a custom liborc_rt archive on disk
bytes	a custom liborc_rt archive in memory
None	no ORC platform

C++ takes Optional<Variant<String, Bytes>>; empty String == "auto", nullopt == None. Linux/ELF only, matching where the ORC platform is wired up today; macOS/Windows ignore the selector as before. A custom runtime must match the LLVM/compiler-rt the extension was built against.

Testing

• New orc_rt tests: custom path (str + Path), in-memory bytes, None (no platform), and bad-type rejection. Path/bytes cases are @skipif non-Linux (the selector only takes effect on ELF).
• Verified locally (aarch64 Linux, LLVM 22): the .so carries the hidden orc_rt_archive symbols in .rodata, absent from the dynamic table; no liborc_rt*.a bundled; C++ exception unwinding across a JIT frame works under auto/path/bytes (proving ELFNixPlatform is genuinely installed).
• Full addon suite: 164 passed, 11 skipped. clang-format / ruff / cmake-format / ASF-header all clean.

[gemini-code-assist]

Code Review

This pull request refactors the ORC JIT session to embed the liborc_rt archive directly into the shared library's .rodata section on Linux/ELF, eliminating the need for on-disk path lookups. It updates the Python and C++ APIs to support a flexible orc_rt selector (supporting embedded, custom path, in-memory bytes, or no platform). The review feedback identifies two issues: first, SetUpOrcPlatform is incorrectly invoked on Windows due to an overly broad preprocessor check (#ifndef __APPLE__), which could cause runtime errors; second, subtracting the two distinct extern pointers orc_rt_archive_end and orc_rt_archive_start to calculate the archive size constitutes undefined behavior in C++ and should be resolved by casting to std::uintptr_t first.

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