[clang] Add _MM_FROUND_TIES_TO_EVEN to avx512fintrin.h (PR #99691)

[via cfe-commits]

hpkfft wrote:

Note that IEEE Std 754-2019 for floating-point arithmetic specifies two rounding direction attributes to nearest: `roundTiesToEven` and `roundTiesToAway`.  Also, it specifies three directed rounding attributes: `roundTowardPositive`, `roundTowardNegative`, and `roundTowardZero`.

This patch follows that naming precedent by assuming round-to-nearest is understood.  If there is a consensus to use a longer macro name (such as `_MM_FROUND_TO_NEAREST_TIES_EVEN` or similar) that is fine too.

https://github.com/llvm/llvm-project/pull/99691