windows-nat: Replace __COPY_CONTEXT_SIZE conditional with __CYGWIN__

windows-nat: Replace __COPY_CONTEXT_SIZE conditional with __CYGWIN__

Replace __COPY_CONTEXT_SIZE conditional with __CYGWIN__

__COPY_CONTEXT_SIZE was added to Cygwin's headers in 2006.

Versions of Cygwin which don't define __COPY_CONTEXT_SIZE are long obsolete.

Also see the thread starting at
https://sourceware.org/ml/gdb-patches/2015-03/msg00989.html for some discussion

Note that __COPY_CONTEXT_SIZE should just be sizeof(CONTEXT) (which is a
platform constant), but isn't due to historical mistakes in Cygwin headers.

gdb/ChangeLog:

2015-06-03  Jon Turney  <jon.turney@dronecode.org.uk>

	* windows-nat.c (do_windows_fetch_inferior_registers)
	(handle_output_debug_string): Replace __COPY_CONTEXT_SIZE
	conditional with __CYGWIN__.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>