arm64: assembler: Switch ESB-instruction with a vanilla nop if !ARM64_HAS_RAS

arm64: assembler: Switch ESB-instruction with a vanilla nop if !ARM64_HAS_RAS

The ESB-instruction is a nop on CPUs that don't implement the RAS
extensions. This lets us use it in places like the vectors without
having to use alternatives.

If someone disables CONFIG_ARM64_RAS_EXTN, this instruction still has
its RAS extensions behaviour, but we no longer read DISR_EL1 as this
register does depend on alternatives.

This could go wrong if we want to synchronize an SError from a KVM
guest. On a CPU that has the RAS extensions, but the KConfig option
was disabled, we consume the pending SError with no chance of ever
reading it.

Hide the ESB-instruction behind the CONFIG_ARM64_RAS_EXTN option,
outputting a regular nop if the feature has been disabled.

Reported-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>