VS: Allow /Fa to set AssemblerListingLocation (#14153)

Generate the default AssemblerListingLocation through the flag map so
that it can be overridden by a user /Fa flag.  Also teach the VS 7-9
generators to map /Fa to AssemblerListingLocation.

While at it, fix the AssemblerListingLocation default value to have a
trailing slash after the configuration name.  This ensures it will be
treated as a directory and not a file name.
Merge topic 'vs-asm-listing-location' into next

7e0c45e VS: Allow /Fa to set AssemblerListingLocation (#14153)