preserve numericness of system() args on Win32

preserve numericness of system() args on Win32

On Windows there's a nasty variation in the meaning of arguments
to Perl's system(), in which a numeric first argument isn't used as
part of the command to run, but instead selects between two different
operations to perform with the command (whether to wait for the command
to complete or not).  Therefore the reduction of argument scalars to
their operative values in the parent process, which was added in commit
64def2aeaeb63f92dadc6dfa33486c1d7b311963, needs to preserve numericness
of arguments on Windows.  Fixes [perl #132633].