Remove Windows ARM64 int8_t workaround in G1

JDK-8248238 added an #ifdef directive to the G1HeapRegionAttr class to work around an MSVC code generation bug on ARM64 (https://github.com/openjdk/jdk/blob/9604ee82690f89320614b37bfef4178abc869777/src/hotspot/share/gc/g1/g1HeapRegionAttr.hpp#L35). The bug has been fixed since Visual Studio 2019 16.8.

As the workaround is specific to Windows-ARM64, and building for this platform already requires the VS 2019 toolset (build tools are still heavily under development), it might make sense to remove the workaround and require a VS version of 16.8 or higher.