[Bug fortran/42954] TARGET_*_CPP_BUILTINS issues with gfortran / Target macros missing (C preprocessor)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42954

--- Comment #50 from Christopher Albert <albert at tugraz dot at> ---
Open for review, comments welcome:
https://github.com/lazy-fortran/gcc/pull/34

This carries attachment 64225 forward. The reserved-namespace-only
behavior from v2 is kept and now covered by regression tests. The main
change is structural, following the plan from comment 19: the x86
macro emission moves out of i386-c.cc into a shared
i386-cppbuiltin.cc linked into the C, C++, and Fortran front ends,
invoked through a new language-neutral TARGET_GENERIC_CPU_CPP_BUILTINS
hook, so cpp.cc no longer duplicates the option-dependent x86 logic.
Other targets can opt in by defining the same hook. check-fortran and
the libgomp Fortran suite are clean; 14 x86 macro-parity scenarios
pass. No rush on this per comment 48; the fork carries it meanwhile.