* system/gc: don't export markStackAndRegisters
* compiler/cgen: unexport internal symbols
As these functions are Nim-specific walkaround against C's optimization
schemes, they don't serve any purpose being exported.
* compiler/cgen: don't export global var unless marked
* compiler/ccgthreadvars: don't export threadvar unless marked
* tests/dll/visibility: also check for exports
This ensure that these changes don't break manual exports.
* compiler/cgen: hide all variables created for constants
* compiler/ccgtypes: don't export RTTI variables
* compiler/ccgexprs: make all complex const static
* nimbase.h: fix export for windows
* compiler/cgen, ccgthreadvars: export variables correctly
For C/C++ variables, `extern` means that the variable is defined in an
another unit. Added a new N_LIB_EXPORT_VAR to correctly export
variables.
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