ebb: ELFv2 EBB fixes

This patches fixes two issues with ELFv2 for EBB: one that the TOC
is not correct materialized and other that EBB setups the wrongly
internal handler.

This patch also adds memory barriers on EBB setup to ensure it is
correct before function returns in thread environments.

	* common/paf-common.h: Add 'mb' memory barrier macro.
	* ebb/ebb-callback-ppc64.S: Fix the TOC materialization for ELFv2.
	* ebb/ebb-sysdep.h: Define ELFv2 entrypoint as null to avoid local
	entries to be used (since EBB handler needs to save all the state
	befora tamper r2).
	* ebb/ebb.c: Fix ELFv2 handler setup and add memory barriers.
ebb: Rewrite test_ebb_pmu_clobber testcase

This patch rewrites EEB GRP clobber testcases to increase predictability
and avoid rely on compiler optimizations.  Current implementation
implements both the GRP setup and check on the symbol assembly routine.