powerpc/perf: Core EBB support for 64-bit book3s

Add support for EBB (Event Based Branches) on 64-bit book3s. See the
included documentation for more details.

EBBs are a feature which allows the hardware to branch directly to a
specified user space address when a PMU event overflows. This can be
used by programs for self-monitoring with no kernel involvement in the
inner loop.

Most of the logic is in the generic book3s code, primarily to avoid a
proliferation of PMU callbacks.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/process.c

@@ -916,7 +916,11 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	flush_altivec_to_thread(src);
 	flush_vsx_to_thread(src);
 	flush_spe_to_thread(src);
+
 	*dst = *src;
+
+	clear_task_ebb(dst);
+
 	return 0;
 }