x86 - add speculation_barrier pattern

x86 - add speculation_barrier pattern

This patch adds a speculation barrier for x86, based on my
understanding of the required mitigation for that CPU, which is to use
an lfence instruction.

This patch needs some review by an x86 expert and if adjustments are
needed, I'd appreciate it if they could be picked up by the port
maintainer.  This is supposed to serve as an example of how to deploy
the new __builtin_speculation_safe_value() intrinsic on this
architecture.

	* config/i386/i386.md (unspecv): Add UNSPECV_SPECULATION_BARRIER.
	(speculation_barrier): New insn.

(backported 5812770a029b1a49b72cabf08b2f202d2d00dc69)

Change-Id: I94d1aacb04c9288a0fbeb38109aeb80a59ca47e6
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263196 138bc75d-0d04-0410-961f-82ee72b054a4