x86: Share definition of __is_canonical_address()

Reduce code duplication by moving canonical address code to a common header
file.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220131072453.2839535-3-adrian.hunter@intel.com