Fix Cygwin gdb build

Fix Cygwin gdb build

Simon pointed out that the windows-nat sharing series broke the Cygwin
build.  This patch fixes the problem, by moving the Cygwin-specific
code to a new handler function.  This approach is taken because this
code calls find_pc_partial_function, which isn't available in
gdbserver.

gdb/ChangeLog
2020-04-16  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_nat::handle_access_violation): New
	function.
	* nat/windows-nat.h (handle_access_violation): Declare.
	* nat/windows-nat.c (handle_exception): Move Cygwin code to
	windows-nat.c.  Call handle_access_violation.

gdbserver/ChangeLog
2020-04-16  Tom Tromey  <tromey@adacore.com>

	* win32-low.cc (windows_nat::handle_access_violation): New
	function.