arm64: Add CNT{P,V}CTSS_EL0 alternatives to cnt{p,v}ct_el0

arm64: Add CNT{P,V}CTSS_EL0 alternatives to cnt{p,v}ct_el0

CNTPCTSS_EL0 and CNTVCTSS_EL0 are alternatives to the usual
CNTPCT_EL0 and CNTVCT_EL0 that do not require a previous ISB
to be synchronised (SS stands for Self-Synchronising).

Use the ARM64_HAS_ECV capability to control alternative sequences
that switch to these low(er)-cost primitives. Note that the
counter access in the VDSO is for now left alone until we decide
whether we want to allow this.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211017124225.3018098-16-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>