VS 16.6 added a `StdOutEncoding` setting for custom commands to tell
MSBuild that the output is encoded as UTF-8. In commit bc877a7e94 (Add
support to indicate UTF-8 custom command pipe output encoding,
2020-04-08) CMake learned to add the setting in anticipation of the VS
16.6 release. However, when 16.6 was released it had a bug in the
implementation of custom tasks with StdOutEncoding enabled that was
exposed by our test suite. In commit 5058fb5401 (VS: Drop
StdOutEncoding with VS 16.6 pending investigation, 2020-05-29) we
disabled the setting pending investigation.
The problem is fixed in VS 16.7 Preview 3, so restore use of the
setting when a VS instance of at least that version is detected.
Fixes: #20769
Add this generator *without* support for specifying the target
architecture in the generator name. cmake-gui will be taught
to provide a field for this, and command-line builds can use -A.
Also, teach this generator to select a default target architecture
based on the host architecture.
Fixes: #18689
Inspired-by: Egor Pugin <egor.pugin@gmail.com>
