Building gcc-16 with GNAT 15 for the bootstrap works, however trying to
build with GNAT 16 the build fails with

[...]
cd ada/gen_il; gnatmake -gnata -gnat2012 -gnatw.g -gnatyg -gnatU
-I/<<PKGBUILDDIR>>/src/gcc/ada gen_il-main
gcc -c -I./ -gnata -gnat2012 -gnatw.g -gnatyg -gnatU
-I/<<PKGBUILDDIR>>/src/gcc/ada -I- 
/<<PKGBUILDDIR>>/src/gcc/ada/gen_il-main.adb
gcc: fatal error: cannot execute ‘gnat1’: posix_spawnp: No such file or
directory
compilation terminated.
gnatmake: "/<<PKGBUILDDIR>>/src/gcc/ada/gen_il-main.adb" compilation error
echo timestamp > s-i386-bt
make[5]: *** [../../src/gcc/ada/Make-generated.in:13: ada/stamp-gen_il]
Error 4
make[5]: *** Waiting for unfinished jobs....
[...]

The unversioned gcc is called, and of course gnat-15 is not installed,
but gnat-16.  Just adding a gcc symlink in the <source pkg>/bin
directory doesn't help.