[llvm] [Runtimes] Allow HandleLibc.cmake to be called multiple times (PR #193540)

[Petr Hosek via llvm-commits]

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@@ -10,42 +10,44 @@
 
 include_guard(GLOBAL)
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petrhosek wrote:

This should be already handled by [`include_guard`](https://cmake.org/cmake/help/latest/command/include_guard.html) so I'd expect this change to be a no-op.

https://github.com/llvm/llvm-project/pull/193540