fuzz: signalBug() / signalBoring()

Instead of a single ASSERT macro, this switches to two new methods:
   - signalBug():    tell afl-fuzz there's a bug caused by its inputs (by crashing)
   - signalBoring(): tell afl-fuzz these inputs are not worth testing (by exiting gracefully)

I'm not seeing any effect on fuzz/s when I just always log verbosely.

signalBug() now triggers SIGSEGV rather than SIGABRT.  This should make it work with catchsegv more easily.

BUG=skia:
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Review URL: https://codereview.chromium.org/1585353002

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