Do not show WoW64 warning message unless CHECK_NOT_WOW64 defined (Win32)

* misc.c (GC_init): Do not show warning message box about WoW64 unless
CHECK_NOT_WOW64 (and GC_WIN32_THREADS); use GetModuleHandle instead of
GetModuleHandleA; check GetModuleHandle result; refine comment;
reformat code.
Resolve MessageBoxA at run-time (if requested) in GC_init (Win32)
(and code refactoring of GC_abort)

* misc.c (GC_win32_MessageBoxA): New static routine (only if MSWIN32,
move code from GC_abort).
* misc.c (GC_init, GC_abort): Replace MessageBoxA with
GC_win32_MessageBoxA call (to resolve "MessageBoxA" at run-time if
DONT_USE_USER32_DLL).