[PATCH 2/4] config.guess, configure.ac: Add detection of IBM z13

[Torbjörn Granlund]

I applied patch 1/4 and 2/4 with modifications.
Please take a look at the repo code when you have the time.

The main change I made to your suggested change is that I added z14 and
z15 to the recognised cpu types.  I also made z13 a fallback for z14 (if
the latter is not understood by tools), and analogously made z14 and z13
fallbacks for z15.

I don't know how relevant the 31-bit ABI is today.  I do not plan to
write any new asm for that, and so did not add special paths for newer
CPUs for that ABI.

-- 
Torbjörn
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