squash! arm64/sve: Signal handling support

Signed-off-by: Dave Martin <Dave.Martin@arm.com>

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Changes since v4
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Miscellaneous:

 * Change inconsistent copy_to_user() calls to __copy_to_user() in
   preserve_sve_context().

   There are already __put_user_error() calls here.

   The whole extended signal frame is already checked for
   access_ok(VERIFY_WRITE) in get_sigframe().