[GR-43070] Track warnings in Native Image build and list them (again) at the very end.

[graalvmbot]

Warnings during image builds can easily be overlooked. This PR counts all warnings and prints a summary again at the end of the build.

Sample output of mx helloworld with 2 warnings during the build.

...
                        1.5s (4.8% of total time) in 66 GCs | Peak RSS: 1.89GB | CPU load: 11.35
------------------------------------------------------------------------------------------------------------------------
Build artifacts:
 /home/christian/oracle/graalvm/graal/substratevm/svmbuild/helloworld (executable)
========================================================================================================================
Finished generating 'helloworld' in 31.0s.

Finished generating 'test' in 20.4s.

The build process encountered 2 warnings.

Hello from native-image!

and optionally, when the -Werror flag is set:

Error: Build failed: Warnings are treated as errors because the -Werror flag is set.
Command '/home/christian/oracle/graalvm/graal/sdk/mxbuild/linux-amd64/GRAALVM_415521A54B_JAVA26/graalvm-415521a54b-java26-26.0.0-dev/bin/native-image -Werror Test -Dllvm.bin.dir=/home/christian/oracle/graalvm/graal/sdk/mxbuild/linux-amd64/LLVM_TOOLCHAIN/bin/' returned non-zero exit status 1.

(outdated screenshot)