AArch64: Use SSBS for CVE_2018_3639 mitigation

AArch64: Use SSBS for CVE_2018_3639 mitigation

The Armv8.5 extensions introduces PSTATE.SSBS (Speculation Store Bypass
Safe) bit to mitigate against Variant 4 vulnerabilities. Although an
Armv8.5 feature, this can be implemented by CPUs implementing earlier
version of the architecture.

With this patch, when both PSTATE.SSBS is implemented and
DYNAMIC_WORKAROUND_CVE_2018_3639 is active, querying for
SMCCC_ARCH_WORKAROUND_2 via. SMCCC_ARCH_FEATURES call would return 1 to
indicate that mitigation on the PE is either permanently enabled or not
required.

When SSBS is implemented, SCTLR_EL3.DSSBS is initialized to 0 at reset
of every BL stage. This means that EL3 always executes with mitigation
applied.

For Cortex A76, if the PE implements SSBS, the existing mitigation (by
using a different vector table, and tweaking CPU ACTLR2) is not used.

Change-Id: Ib0386c5714184144d4747951751c2fc6ba4242b6
Signed-off-by: Jeenu Viswambharan <[email protected]>