Changes for Customer W; also suitable for building on an Iyonix.
Ben Avison authored
Detail:
  * No longer uses aasm to build h.swis.
  * (Only) compatible with new C compilers: assumes const static data is
    placed in separate read-only areas, and suitable command-line switch
    is used to ensure that library static data is never placed in a zero-
    init area.
  * Stack extension code now thread-safe.
  * Heap thread-safety code (in clib) now uses SWP instruction test results
    from stack extension code (in kernel) - as a side effect, the exported
    symbol _swp_available now exists, if you need to use it elsewhere.
  * Slightly closer to having top-bit-set heap addresses working.
Admin:
  Tested in a Tungsten build, and with Customer W's test suite.

Version 5.48. Tagged as 'RISC_OSLib-5_48'
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