Fix 'replacement operator delete cannot be inline' GCC warning (Cygwin)

Fix 'replacement operator delete cannot be inline' GCC warning (Cygwin)
(Cherry-pick commit 057011d from 'release-7_6' branch.)

This commit also eliminates
"operator delete is missing exception specification throw()" compiler
warning.

* gc_cpp.cc (operator delete): Define unconditionally (i.e. for Cygwin
too).
* include/gc_cpp.h [__CYGWIN__]: Do not include "new" header.
* include/gc_cpp.h [__CYGWIN__] (operator delete): Remove (as
replacement function 'operator delete' cannot be declared 'inline').