I tried to debug it before, see this thread: https://forum.ada-lang.io/t/gprconfig-for-bare-metal-target/4409
I’m not sure how GNAT Pro build their cross-compilers, but I’m building a bare metal sparc64 cross-compiler without any Ada runtime, which is also missing a few other tools like gnatls. I think gprconfig may be missing some logic to correctly identify such cross-compilers. It finds sparc64 C and Asm compilers, but refuses to find Ada compiler. There is no indication why. The Ada compiler is definitely there, because I can use it to build sparc64 object files, i.e. when building Ada runtime. This is the reason I had to create a custom *.cgpr file to workaround these issues.
I’m not sure if this is a bug with gprconfig or if there is a different strategy for building Ada cross-compilers. Following your suggestion, below is the command invoked by gprbuild:
PATH="${HOME:?}/gcc-15.2.0_native/bin:${HOME:?}/gcc-15.2.0_sparc64-unknown-elf/bin" \
/home/user/gcc-15.2.0_native/bin/gprconfig -vv --batch \
-o /tmp/out --target=sparc64-unknown-elf \
--config=c,,,, --config=ada,,/home/user/bare_runtime,,
...
gprconfig: can't find a toolchain for the following configuration:
gprconfig: language 'ada', target 'sparc64-unknown-elf', runtime '/home/user/bare_runtime'