_M_ARM64EC cleanups, v2

[StephanTLavavej]

Followups to GH-1344 requested by @cbezault.

• xutility
  • Extract && !defined(_M_ARM64EC) to the end for clarity. There's no behavioral change here (_M_ARM64EC can be defined only when _M_X64 is defined), it just separates the major architectures where we want to enable the vectorized algorithms (x86, x64) from the flavors where they should be excluded due to intrinsic unavailability.
• stl/msbuild/stl_base/msvcp.settings.targets
• stl/msbuild/stl_atomic_wait/stl_atomic_wait.files.settings.targets
  • Add and remove semicolons for consistency.
• vector_algorithms.cpp
  • Adjust preprocessor guards. We never build this for /clr:pure, so that can be a hard #error. However, we do need to make this file a no-op for _M_ARM64EC. Extract that, like xutility above.

I tested this with MSVC-internal builds of x86, x64, arm, arm64, and chpe (there is no dedicated arm64ec build).