IA-16: bump __ia16__ to 20240218L

IA-16: fix (?): disable IPA SRA optimization over __far stuff

Partly addresses #144 .

Fix build on macOS host (?)

As proposed at https://github.com/tkchia/build-ia16/issues/28#issue-1787527850 .

IA-16: bump __ia16__ to 20230219L

IA-16: fix: `#include_next <limits.h>' broken on elks-libc rebuild

IA-16: add test for -finstrument-functions-simple

IA-16: add experimental -mdosx32 option

See #129 .

Partially backport -Wprio-ctor-dtor option from (?) GCC 8.x

This is a partial backport of a commit for newer GCCs (see
8cff065), which adds a
-Wprio-ctor-dtor option.  The option controls warnings about
__attribute__((constructor( ))) & __attribute__((destructor( )))
priority numbers.

IA-16: pare down built-in specs for ELKS

IA-16: bump __ia16__ to 20221121L