Irix: MIPSpro 7.4 compiler has broken memcmp.

Irix: MIPSpro 7.4 compiler has broken memcmp.

(Supposedly 7.4.1m is okay in this regard, and there is also 7.5.)

The commit that 'broke' Perl build on IRIX was 57620943
which of course wasn't to blame, the IRIX compiler was.
(Symptom: miniperl crashing, can be narrowed to just /[[:alpha:]]/)

Furthermore: IRIX build was also earlier broken for a long time by
another problem (in the preprocessor), see for example perl #33849.
(Symptom: compile failing in gv.c RvDEEPCP)

Summary: building Perls on IRIX needs either this very commit,
or undoing the 57620943 (for the memcmp brokenness), and possibly
applying both the c297d531 and 08c5d564 (for the cpp brokenness).
(The latter were applied between 5.10.0 and 5.10.1.)