Build fix
Ben Avison authored
Detail:
  Defines UXTH and SXTH as macros. This means we can assemble the output of
  cc -S -cpu 6 even using current versions of objasm that are limited to the
  v5TEJ instruction set. It is expected that objasm will ignore these macros
  if/when it gains v6 instruction set support.
Admin:
  Tested - it works.
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