headers: properly decorate attributes

headers: properly decorate attributes

As pointed out here:
https://cygwin.com/ml/cygwin/2014-07/msg00371.html

any use of __attribute__ in a header that can be included by a user
should be namespace-safe, by decorating the attribute arguments with __
(while gcc does a lousy job at documenting it, ALL attributes have a __
counterpart, precisely so that public headers can use attributes without
risk of collision with macros belonging to user namespace).

* include/pthread.h: Decorate attribute names with __, for
namespace safety.
* include/cygwin/core_dump.h: Likewise.
* include/cygwin/cygwin_dll.h: Likewise.
* include/sys/cygwin.h: Likewise.
* include/sys/strace.h: Likewise.