Remove support for 8 byte tear free read/write on 32-bit

Remove support for 8 byte tear free read/write on 32-bit

The macro for enabling single-copy atomicity on i586+ when using
GCC has been incorrect since 2017 (commit e8fdbd58f) without any
complaints, and getting it to work is non-trivial.

Getting this to work reliably require C11 atomics, which in turn
also bumps the required MSVC version. For now, simply remove the
attempted support which doesn't work anyways.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reported-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Suggested-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAKZiRmycHOOJyEPc9FUss1_69_U62WoSx32jT7wyES-YkStZKA@mail.gmail.com
Discussion: https://posrgr.es/m/CA+hUKGKFvu3zyvv3aaj5hHs9VtWcjFAmisOwOc7aOZNc5AF3NA@mail.gmail.com