jit: precompute symbol offsets, add x86-64 GOT relaxation, and include lightweight JIT benchmark tool

[methane]

Motivation

• Reduce emit-time overhead by avoiding repeated popcount walks when resolving GOT/trampoline slots.
• Improve generated code quality by enabling an x86-64 relaxation that can replace RIP-relative loads with immediates when safe.
• Provide a lightweight local benchmark to quickly smoke-test JIT changes before running larger suites.

Description

• Added an offsets array to symbol_state and implemented initialize_symbol_state to precompute ordinal-to-slot indices and region sizes.
• Replaced the popcount-based slot index calculation in get_symbol_slot with a direct lookup using state->offsets[ordinal].
• Replaced manual size accumulation loops with calls to initialize_symbol_state and simplified padding/alignment arithmetic for code/data/GOT regions.
• Extended patch_x86_64_32rx to relax certain mov reg, [rip + AAA] sequences into immediate mov reg, imm using the C7 /0 encoding while preserving REX.W handling.
• Updated Tools/jit/README.md with a "Measuring JIT performance" section and added Tools/jit/benchmark.py as a small JIT-focused benchmark driver.

Testing

• Built CPython with --enable-experimental-jit and the build completed successfully.
• Ran the new benchmark smoke test via Tools/jit/benchmark.py to validate jit_available/jit_enabled reporting and collected timings, which completed without error.
• Ran the project's test suite focusing on JIT-related tests with make test, and the executed tests passed.

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