feat: add Linux UAPI headers formula

[MeteorsLiu]

Add a standalone Linux UAPI headers formula with Linux-aware release ordering.

The implementation includes:

• Export headers with make headers_install, LLVM=1, and an explicit target ARCH for Linux amd64 and arm64.
• Select the formula starting at Linux v5.7, where upstream introduced LLVM=1.
• Check exported syscall numbers and UAPI types with target-specific Clang consumer tests.
• Add a module-local comparator that orders matching -rcN tags before final releases and -dontuse variants before their normal RC, while retaining GNU numeric ordering for historical tags.
• Validate all 946 historical tags against an independent numeric ordering model, plus latest-version selection, the v5.7 formula boundary, and dependency version selection.

This keeps header exports architecture-correct and prevents unversioned requests from selecting an older RC after its final release. Next-cycle RCs remain newer than the preceding final release; LLAR's global comparator and Git reference handling are unchanged.

[fennoai]

Reviewed the complete Linux UAPI formula diff. The build and consumer-test paths are internally consistent, but the module needs Linux-aware tag ordering to avoid selecting release candidates over final releases.

Validation: inspected LLAR bd473ea21632d57a4ed21e54144a55d0ee6c1872, Linux v5.7 Kbuild behavior, and reproduced the default comparator ordering. A local llar test ./torvalds/linux@v5.7 --os linux --arch amd64 compiled and loaded the formula, then stopped because this runner does not have Clang installed.

View job run

[fennoai]

[P2] Define Linux release-aware tag ordering

Without a _cmp.gox, LLAR uses GNU version comparison, which orders a final tag before its RC suffix (for example, v7.2 < v7.2-rc7). As a result, whenever the newest release has no next-cycle RC yet, an unversioned torvalds/linux request resolves to the release candidate instead of the final release. Add a Linux-aware comparator that treats -rcN as a prerelease of the matching final version while still handling the repository's complete historical tag set.