Fix potential buffer overflow in makecontext trampoline

Fix potential buffer overflow in makecontext trampoline

	glibc's tst-makecontext2 testcase uncovered a bug in
	__cont_link_context.  If the function misses to reserve
	shadow space for the calls to setcontext/cygwin_exit,
	both functions could overwrite memory beyond the stack
	configured in uc_stack.

        * exceptions.cc (__cont_link_context): x86_64: align stack and reserve
        shadow space for subsequent function calls, otherwise suffer potential
        buffer overflow.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>