[libc-commits] [libc] [libc][math][c23] Add acospif16() function (PR #134664)

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amemov wrote:

I implemented a workaround for older versions of MPFR. Right now the acospif16 fails with some values that approach `-1`

Not exactly sure where I messed up - with polynomials in `acospif16.cpp` or with rounding in `MPCommon.cpp` workaround that I wrote 

https://github.com/llvm/llvm-project/pull/134664