[clang] [clang][mlibc] Predefine `__mlibc__` for mlibc targets (PR #225654)

[via cfe-commits]

https://github.com/lzcunt created https://github.com/llvm/llvm-project/pull/225654

Pretty much all users of mlibc targets currently vendor this or similar patches and having it upstream would be useful.

>From 5f5e7edf1969cbec97b3af9f67509c75404bf235 Mon Sep 17 00:00:00 2001
From: lzcunt <lzcunt at rawr.fun>
Date: Wed, 23 Sep 2026 12:18:59 +0300
Subject: [PATCH] [clang][mlibc] Predefine `__mlibc__` for mlibc targets

---
 clang/lib/Basic/Targets/OSTargets.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/clang/lib/Basic/Targets/OSTargets.h b/clang/lib/Basic/Targets/OSTargets.h
index fb5c17cefc004e..36354668de69cb 100644
--- a/clang/lib/Basic/Targets/OSTargets.h
+++ b/clang/lib/Basic/Targets/OSTargets.h
@@ -396,6 +396,9 @@ class LLVM_LIBRARY_VISIBILITY LinuxTargetInfo : public OSTargetInfo<Target> {
     } else {
         Builder.defineMacro("__gnu_linux__");
     }
+    if (Triple.getEnvironmentName() == "mlibc") {
+      Builder.defineMacro("__mlibc__");
+    }
     if (Opts.POSIXThreads)
       Builder.defineMacro("_REENTRANT");
     if (Opts.CPlusPlus)