Use bigobj for release MSVC builds

Previously it was used only on debug builds, but recent
additions to variant_call.cpp have made that unit too
large even in release_debug.

It was originally set to debug only because using it
breaks mingw, so this only reinstates it for MSVC where
it shouldn't have any drawbacks (famous last words).

Test-Information:
Builds for me with
scons -j 40 platform=windows target=release_debug
and my game then runs from it.

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