compiler.h BUILTIN_SUBCLL_BROKEN workaround can be disabled on xcode 15 and up

Under issue #1659 (closed) we added a workaround in compiler.h for an Apple clang bug where its __builtin_subcll() was broken in Xcode 14. We have a report in that issue that Xcode 15 and 16 do not have this bug:

"I just checked Xcode 15.0 and the latest version (16.3), and the fix is present in the corresponding swiftlang/llvm-project branches, and x86_64-softmmu tests don't hang with the workaround disabled."

So we can update our workaround to check for __clang_major == 14 rather than >= 14, since only Xcode 14 had this problem.

Edited Jul 11, 2025 by Peter Maydell

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