aarch64 - Set the mode for the unspec in speculation_tracker insn.

aarch64 - Set the mode for the unspec in speculation_tracker insn.

The speculation tracker insn in my recent patch set for CVE-2017-5753
was missing a mode on the UNSPEC.  Although this didn't break the
build, it did cause an unnecessary warning from the MD parsing
mechanism that I missed at the time.  It's a trivial fix, as follows:

	* config/aarch64/aarch64.md (speculation_tracker): Set the mode for
	the UNSPEC.

(backported 1ce555a208bf8e3422eadd9c1e3eb0b1b120a81b)

Change-Id: I525ae85eeafc2e9f42b365957abdb7b494a286c2
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@265849 138bc75d-0d04-0410-961f-82ee72b054a4