AArch64 - Add new option -mtrack-speculation

This patch doesn't do anything useful, it simply adds a new command-line
option -mtrack-speculation to AArch64.  Subsequent patches build on this.

	* config/aarch64/aarch64.opt (mtrack-speculation): New target option.

From-SVN: r263171

gcc/ChangeLog

 2018-07-31  Richard Earnshaw  <rearnsha@arm.com>
 
+	* config/aarch64/aarch64.opt (mtrack-speculation): New target option.
+
+2018-07-31  Richard Earnshaw  <rearnsha@arm.com>
+
 	* config/aarch64.md (unspecv): Add UNSPECV_SPECULAION_BARRIER.
 	(speculation_barrier): New insn.
 

gcc/config/aarch64/aarch64.opt

@@ -214,3 +214,7 @@ Target RejectNegative Joined Enum(sve_vector_bits) Var(aarch64_sve_vector_bits)
 mverbose-cost-dump
 Common Undocumented Var(flag_aarch64_verbose_cost)
 Enables verbose cost model dumping in the debug dump files.
+
+mtrack-speculation
+Target Var(aarch64_track_speculation)
+Generate code to track when the CPU might be speculating incorrectly.