Squelch a noreturn warning on AIX. [skip ci]

Squelch a noreturn warning on AIX. [skip ci]

GCC 8.3.0 on AIX 7.1 produced a warning for every file that has a call
to nd_trunc_longjmp():

In file included from ./cpack.c:39:
./extract.h: In function 'nd_trunc_longjmp':
./extract.h:586:1: warning: 'noreturn' function does return

As far as longjmp(3) man page and <setjmp.h> header go, longjmp() is as
noreturn in AIX as everywhere else, but not in a way that GCC can tell.

Disabling -Wsuggest-attribute=noreturn and -Wmissing-noreturn had no
apparent effect, so instead of using diag-control.h add an infinite loop
on AIX to squelch the warning.