codegen: one target-OS classifier (follow-up to #12247's triple-spelling fix) #12249

Hardening follow-up to #12247. #12247 is correct: the heap floor is a checked lower bound, and any address outside the range takes the runtime slow path.

1. Hard-coded floor. target_layout.rs writes the floor as the literal 0x10_0000. Use crate::runtime_abi::RECEIVER_HANDLE_FLOOR instead, as receiver_range.rs:37 already does.

2. Hand-listed test targets. heap_address_floor_covers_emitted_triples lists target names by hand and misses wearos, windows-arm64, windows-x86_64, linux-x86_64-musl, windows-winui and wasi. Expose one TARGET_NAMES next to resolve_target_triple (codegen/helpers.rs:994) and loop over it.

3. One OS classifier. Several places classify the target OS by ad hoc substring matching on the triple:

   • helpers.rs:155
   • expr/literals_vars.rs:223
   • inprocess/function_layout.rs:27
   • gc_map.rs:1192
   • commands/compile/post_link.rs:87

   fix(codegen): treat LLVM macOS and visionOS triples as low-address heap targets #12247 fixed the same class of bug, where LLVM and rustc spell the OS differently in a triple. Add one target_os_of(triple) classifier that mirrors rustc's target_os, route these sites through it, and test it against cfg!(target_os) for the default triple.