MSVC: update arm64ec default flags and lib

moyo1997requested to merge
moyo1997/cmake:msvc-arm64ec into master

Update the support for arm64ec added by !5708 (merged).

Add softintrin.lib to CMAKE_C_STANDARD_LIBRARIES_INIT.

Remove /D_ARM64EC_WORKAROUND from the flags as it was only ever needed in preview versions of VS.

Issue: #21724

Edited by Brad King

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