fix #15955: avoid duplicate symbol errors with --app:staticlib by using weak symbols

[timotheecour]

fix #15955 by introducing -d:nimLinkerWeakSymbols which generates weak symbols via __attribute__((weak)), see tests

note

currently fails on windows: see https://dev.azure.com/nim-lang/255dfe86-e590-40bb-a8a2-3c0295ebdeb1/_apis/build/builds/12851/logs/89

• https://stackoverflow.com/a/51970423/1426932 which covers __attribute__((weak)) and static linking
• https://stackoverflow.com/questions/2290587/gcc-style-weak-linking-in-visual-studio which discusses MSVC++'s analog to __attribute__((weak)), with either __declspec(selectany) or
  #pragma comment(linker, "/alternatename:_pWeakValue=_pDefaultWeakValue")
• https://docs.microsoft.com/en-us/cpp/error-messages/compiler-errors-1/compiler-error-c2496?view=msvc-160
• https://devblogs.microsoft.com/oldnewthing/20200731-00/?p=104024 /ALTERNATENAME linker switch

[Araq]

Succeeded by #19235.