x86-64/setcontext: Pop the pointer into %rdx after syscall

To prepare for shadow stack support, pop the pointer into %rdx after
syscall and use %rdx, instead of %rsi, to restore context.  There is
no functional change.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

	* sysdeps/unix/sysv/linux/x86_64/setcontext.S (__setcontext):
	Pop the pointer into %rdx after syscall and use %rdx, instead
	of %rsi, to restore context.