[gmx-users] MSVC or later now supports AVX2 and AVX_512

[Tatsuro MATSUOKA]

In gmxSimdFlags.cmake, it is described :
# no AVX2-specific flag for MSVC yet

However, at least MSVC 2017 and 2019, /arch:AVX2 is added.
If I modify /arch:AVX to /arch:AVX2, gromacs can be complied with avx 256.

Tatsuro