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
1 file changed