x86-64/swapcontext: Restore the pointer into %rdx after syscall

To prepare for shadow stack support, restore 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/swapcontext.S (__swapcontext):
	Restore the pointer into %rdx, after syscall and use %rdx,
	instead of %rsi, to restore context.