[libc] [llvm] [libc] Remove obsolete LIBC_HAS_BUILTIN macro (PR #86554)

[Joseph Huber via llvm-commits]

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@@ -21,6 +21,11 @@
 
 using namespace LIBC_NAMESPACE;
 
+// Workaround for missing __has_builtin in < GCC 10.
+#ifndef __has_builtin
+#define __has_builtin 0
+#endif
+
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jhuber6 wrote:

Needs to be above the `#include`s.

https://github.com/llvm/llvm-project/pull/86554