system.h: assume OS400 is always built with ILEC compiler

system.h: assume OS400 is always built with ILEC compiler

Prior to this change the OS400 types were only defined when __ILEC400__.
That symbol is only defined by IBM's C compiler and not their C++
compiler, which led to missing types when users on OS400 would compile a
C++ application that included curl.

The IBM C and C++ compilers are the only native compilers on the
platform.

Assisted-by: Jon Rumsey
Reported-by: John Sherrill
Fixes https://github.com/curl/curl/issues/10305
Closes https://github.com/curl/curl/pull/10329