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The speculation barrier feature (`FEAT_SB`) was introduced with and
made mandatory in the Armv8.5-A extension. It was retroactively made
optional in prior extensions, but the checks in our code-base do not
reflect that, assuming that it is only available in Armv8.5-A or later.

This change introduces the `ENABLE_FEAT_SB` definition, which derives
support for the `sb` instruction in the assembler from the feature
flags passed to it. Note that we assume that if this feature is enabled
then all the cores in the system support it - enabling speculation
barriers for only a subset of the cores is unsupported.
Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: I978ed38829385b221b10ba56d49b78f4756e20ea