meson: Fix runtime CPU detection with MSVC
With GCC, Clang, ICC, etc, we differentiate between 'may support this SIMD' and 'presume we have this SIMD' by checking whether the SIMD / intrinsics can be compiled by the compiler as-is (presume) or with SIMD cflags (may have). With MSVC, the compiler will always build SIMD/intrinsics targeting all specific instruction sets supported by that version of the compiler. No special arguments are ever needed. If runtime CPU detection is not disabled, we must always assume that we only 'may have' it.