22.1.8.3: the Apple rows, whose payload carries no compiler runtime for the platform

[Sunrisepeak]

The official LLVM macOS payload builds libclang_rt.osx.a and no ios or iossim archive (measured on llvm 22.1.8, mcpp-community/mcpp#630), and clang's Darwin driver links the archive only when the file exists. A program that reaches an availability check then fails at link with __isPlatformVersionAtLeast undefined, and nothing said so earlier.

This release adds upstream's Darwin selection under cfg(os = "ios") (the six atomic-flag and fence units the M-profile rows withhold) and the x86_64/ directory for the x86_64 simulator; os_version_check.c, which defines the routine, is already in the package-wide list and compiles to nothing where __APPLE__ is not defined. The macOS row is not listed: its payload carries the archive.

CI gains a macOS job that builds both simulator rows and reads ___isPlatformVersionAtLeast out of the archive, since a build that succeeds can leave the routine out.

An iOS application declares the package beside its C++ standard library package:

[target.'cfg(os = "ios")'.dependencies]
llvm.compiler-rt-builtins = "22.1.8.3"
llvm.libcxx               = "22.1.8.1"

Design record: mcpp-community/mcpp .agents/docs/2026-09-13-630-what-a-framework-still-hits-in-the-engine.md, §5.