[llvm] [OpenMP] Use the `libc` malloc for AMDGPU if available (PR #151241)

[Matt Arsenault via llvm-commits]

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@@ -50,7 +50,7 @@ namespace {
 ///
 ///{
 extern "C" {
-#ifdef __AMDGPU__
+#if defined(__AMDGPU__) && !defined(OMPTARGET_HAS_LIBC)
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arsenm wrote:

Can you keep this logic fully in cmake by splitting out the malloc / free to a separately linked object 

https://github.com/llvm/llvm-project/pull/151241