Port pdumper mmap to AIX

* src/pdumper.c (needs_mmap_retry_p) [_AIX]: Return true.
Problem observed on AIX 7.1 and 7.2 in GCC compile farm.
1 job for master in 54 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #3201
test-all

00:54:08

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1