tccgen/win32: let __declspec(dllimport) imply extern

Also, retain storage qualifiers in type_decl, in particular
also for function pointers.  This allows to get rid of this
very early hack in decl()
    type.t |= (btype.t & VT_STATIC); /* Retain "static". */
which was to fix the case of
    int main() { static int (*foo)(); ...

Also:
- missing __declspec(dllimport) is an error now
- except if the symbol is "_imp__symbol"
- demonstrate export/import of data in the dll example (while
  'extern' isn't strictly required with dllimport anymore)
- new function 'patch_storage()' replaces 'weaken_symbol()'
  and 'apply_visibility()'
- new function 'update_storage()' applies storage attributes
  to Elf symbols.
- put_extern_sym/2 accepts new pseudo section SECTION_COMMON
- add -Wl,-export-all-symbols as alias for -rdynamic
- add -Wl,-subsystem=windows for mingw compatibility
- redefinition of 'sym' error for initialized global data

tcc.c

@@ -117,13 +117,14 @@ static const char help2[] =
     "-Wl,... linker options:\n"
     "  -nostdlib                     do not link with standard crt/libs\n"
     "  -[no-]whole-archive           load lib(s) fully/only as needed\n"
+    "  -export-all-symbols           same as -rdynamic\n"
     "  -image-base= -Ttext=          set base address of executable\n"
     "  -section-alignment=           set section alignment in executable\n"
 #ifdef TCC_TARGET_PE
     "  -file-alignment=              set PE file alignment\n"
     "  -stack=                       set PE stack reserve\n"
     "  -large-address-aware          set related PE option\n"
-    "  -subsystem=[console gui]      set PE subsystem\n"
+    "  -subsystem=[console/windows]  set PE subsystem\n"
     "  -oformat=[pe-* binary]        set executable output format\n"
     "Predefined macros:\n"
     "  tcc -E -dM - < nul\n"