x32: Avoid unsigned long when installing fast tracepoint jump pads

x32: Avoid unsigned long when installing fast tracepoint jump pads

We're casting through unsigned long to write a 64-bit immediate
operand of movabs (the comment said movl, but that was incorrect).
The problem is that unsigned long is 32-bit on x32, so we were writing
fewer bytes than necessary.

Fix this by using an 8 byte memcpy like in other similar places in the
function.

gdb/gdbserver/ChangeLog:
2016-08-19  Pedro Alves  <palves@redhat.com>

	* linux-x86-low.c (amd64_install_fast_tracepoint_jump_pad): Fix
	comment.  Use memcpy instead of casting through unsigned long.