[Win32] Set $Config{installsitescript} to a sane value.

[sisyphus]

Both "win32/config.gc" and "win32/config.vc" currently assign the empty string to installsitescript.
This led to the failing test in Module-Build-Tiny-0.048 reported at Perl-Toolchain-Gang/module-build-tiny#35.

Both installbin and installsitebin are currently assigned the value ~INST_TOP~~INST_VER~\bin~INST_ARCH~ and this PR assigns that same value to installsitescript - which fixes that failing Module-Build-Tiny test.

This PR also corrects installscript (which was previously missing the ~INST_ARCH~ appendage) so that it, too, becomes identical to the other three.

AIUI, Strawberry Perl is unaffected by anything we change here, as their build procedure sets these Config values independently of the win32/config.gc specification.

I'm not sure whether the 2 "site" variants should specify \site\bin instead of just \bin.
I've left it as just \bin, mainly because perl\site\bin is generally (always?) non-existent when a fresh build of Windows perl is installed, and people might not realize that they have to add that non-existent directory to the path ... and then spend hours trying to work out why some subsequently installed module has apparently failed to install its "scripts".
Besides, the current setting for installsitebin has not led to any report of problems (AFAIK), so why change it ?
I am, of course, open to suggestions wrt this.

[Leont]

I'm not sure whether the 2 "site" variants should specify \site\bin instead of just \bin.

Both options are valid. I guess the most sensible default is the one that's most likely to work with no further effort so probably what this PR does is the right thing. If someone doesn't like it they can just override it.