[klibc] REAMDE.klibc: Update architecture status

* Tests are passing for arm-thumb, parisc and sh
* There are some test failures on alpha and ia64, but the shared
  library works on ia64
* arm26 and v850 have been removed for Linux so we have no expectation
  of ever supporting them

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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