fix twrong_refcounts in nim cpp mode

[timotheecour]

twrong_refcounts was failing in cpp mode because getRefcount from

lib/system/gc.nim:131:50:proc internRefcount(p: pointer): int {.exportc: "getRefcount".} =

gets mangled as C++ in nim cpp:

nm stdlib_system.cpp.o|grep getRefcount
000000000000d140 T __Z11getRefcountPv

whereas on the import side:

proc getRefcount*[T](x: ref T): int {.importc: "getRefcount", noSideEffect,
  deprecated: "the refcount never was reliable, the GC does not use traditional refcounting".}

gets the declaration mangled as extern C, not extern C++, which points to a limitation we should address soon, such as ability to have extern C exportc procs in nim cpp ( a few syntaxes were discussed; will be addressed in future PR's)

So this PR introduces a workaround until we get ability to have extern C exportc procs in nim cpp

after this PR and #10310 there'll be 1 2 last failing tests before we can enable remove allow_failures for env: NIM_COMPILE_TO_CPP=true:

• tests/niminaction/Chapter8/sdl/sdl_test.nim
• tests/objects/tobjcov.nim