Fix libjava build failure with glibc-2.26, replacing 'struct ucontext' with
ucontext_t.  Ok for the gcc-5 and gcc-6 branches?  Doesn't apply to 7 and trunk,
because libjava is removed.

Matthias

	* include/x86_64-signal.h (HANDLE_DIVIDE_OVERFLOW): Replace
	'struct ucontext' with ucontext_t.
	* include/i386-signal.h (HANDLE_DIVIDE_OVERFLOW): Likewise.
	* include/s390-signal.h (HANDLE_DIVIDE_OVERFLOW): Likewise.