fix: Add fallback byteswap/endian implementations for older abseil versions

[yharby]

Summary

• Add fallback byteswap implementations using compiler intrinsics for older abseil versions
• Add compile-time endianness detection via __BYTE_ORDER__ macros
• Maintain backwards compatibility while still working with newer abseil

Problem

s2geometry v0.13.1 uses absl::byteswap() and absl::endian in src/s2/util/endian/endian.h, which were only added in abseil LTS 2025.01.27. This causes compilation failures when building with older abseil versions that many package managers still provide:

error: 'byteswap' is not a member of 'absl'
error: 'absl::endian' has not been declared

Solution

This PR adds fallback implementations that:

1. Use __builtin_bswap16/32/64 compiler intrinsics (GCC/Clang) with manual fallbacks for other compilers
2. Detect endianness at compile time using __BYTE_ORDER__ macros
3. Are transparent - when newer abseil is available, the code still works

Testing

• Tested locally with duckdb-geography extension build (all 170 tests pass)
• All s2geometry CI tests pass on ubuntu-22.04, ubuntu-latest, macos-latest-large, macos-latest-xlarge

Fixes #489

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