[libc] Build fuzzing tests in pre-merge CI tests (#185018)
At the moment, no CI job tests whether the fuzzing tests build
correctly.
This patch adds the build of fuzzing tests to the pre-merge CI job.
Only two configurations have it enabled for now. The none-eabi
configurations seemingly do not support it because in their cmake
configs compiler-rt is not enabled, hence libFuzzer isn't built. I did
not dig too much to understand why that is, preferring to just leave it
disabled for these configurations. For the remaining ones that seem to
support it, I selected one x86 and one aarch64.
In addition, it removes one outdated comment about the build type used
and changes the action to run on all branches, not only on PRs that
target main.
If we limit it to run only on PRs to the main branch, it will not run on
stacked PRs. I believe it is also okay to run it on PRs to release
branches. Therefore it is just easier to remove the limit altogether.
