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Detail:
  Sync the disc copies of the compiler's internal headers from cc 5.89.
  Add <stdalign.h> and <stdnoreturn.h> and <uchar.h> in support of C18.
  Now the compiler sets __STDC_VERSION__ appropriately on the nightly builds it
  is no longer necessary to use predefine 'DDE' to enable those additions.

Version 6.15. Tagged as 'RISC_OSLib-6_15'