[clang] [libc][libm][GPU] Add missing vendor entrypoints to the GPU version of `libm` (PR #66034)

[Joseph Huber via cfe-commits]

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+//===-- Implementation of the GPU lroundf function
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jhuber6 wrote:

This and some other headers need to be reformatted.

https://github.com/llvm/llvm-project/pull/66034