msvc: Enable GC

Msvc only declares the semi-standard _bss_start and _end variables (for compatibility I guess), but they do not point to the actual bss section used. That is appended to the end of the data section by the linker so it's not possibly to look it up (or I found no way). However putting all statics in a named section and then querying the executable to figure out where the section is, and use that for the GC, seems to work just fine.
Also see #625 - a couple of tests show the behaviour is now roughly the same as gcc builds.
@stinos , sorry if this was only hinted, not said explicitly: I proposed via-void* workaround not as a way to do it differently than you did, but as a way to not add any more lines to the code to work the issues around. So, extra cast could be added straight to this line, and there certainly no need to have the #ifdef above - we have them enough already, going to have more, so any case where it can be avoided is rather be pursued. Thanks.