introduce mingw assembler flavour to ARM too

[Sashan]

There is no mingw flavor avaiable for arm perlasm files. We currently use win64 flavour for arm perlasm files on windows. This is a quic workaround for #26415. We need to get it fixed with introduction of microsoft assembler for Win64 on ARM. We currently don't build that. This is something what should be done in 3.6 more work is required in build management.

The existing win64 flavor we use for arm should be renamed to mingw64.