[libFuzzer] Add attribute noinline on Fuzzer::ExecuteCallback().

The inlining of this function needs to be disabled as it is part of the
inpsected stack traces. It's string representation will look different
depending on if it was inlined or not which will cause it's string comparison
to fail.

When it was inlined in only one of the two execution stacks,
minimize_two_crashes.test failed on SystemZ. For details see
https://bugs.llvm.org/show_bug.cgi?id=49152.

Reviewers: Ulrich Weigand, Matt Morehouse, Arthur Eubanks

Differential Revision: https://reviews.llvm.org/D97975