MSVC: update arm64ec default flags and lib Update the support added by commit 4ea3a88625 (MSVC: Add support for targeting ARM64EC, 2020-12-30, v3.20.0-rc1~121^2). 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