build: Make G_FUZZING constexpr, require -DBUILD_FOR_FUZZING=ON to fuzz

g_fuzzing is used inside Assume at runtime, causing significant overhead in hot paths. See https://github.com/bitcoin/bitcoin/issues/31178

One could simply remove the g_fuzzing check from the Assume, but this would make fuzzing a bit less useful. Also, it would be unclear if g_fuzzing adds a runtime overhead in other code paths today or in the future.

Fix all issues by making G_FUZZING equal to the build option BUILD_FOR_FUZZING, and for consistency in fuzzing, require it to be set when executing any fuzz target.

Fixes https://github.com/bitcoin/bitcoin/issues/31178

Temporarily this drops fuzzing from two CI tasks, but they can be re-added in a follow-up with something like https://github.com/bitcoin/bitcoin/pull/31073