This is basically the minimum viable signal handler for Wasm bounds checks.
It includes the TLS check and the fine grained instructions checks. These
two checks provide most of the safety for the signal handler. Future CLs will
add code range and data range checks for more robustness.

The trap handling code and data structures are all in src/trap-handler, with
the code that actually runs in the signal handler confined to
src/trap-handler/signal-handler.cc.

This changes adds a new V8 API that the embedder should call from a signal
handler that will give V8 the chance to handle the fault first. For hosts that
do not want to implement their own signal handler, we include the option to
install a simple one. This simple handler is also used for the tests.

When a Wasm module is instantiated, information about each function is passed
to the trap handler, which is used to classify faults. These are removed during
the instance finalizer.

Several future enhancements are planned before turning this on by default.
Obviously, the additional checks will be added to MaybeHandleFault. We are
also planning to add a two-level CodeObjectData table that is grouped by
isolates to make cleanup easier and also reduce potential for contending on
a single data structure.

BUG= https://bugs.chromium.org/p/v8/issues/detail?id=5277

Review-Url: https://codereview.chromium.org/2371833007
Cr-Original-Commit-Position: refs/heads/master@{#43523}
Committed: https://chromium.googlesource.com/v8/v8/+/a5af7fe9ee388a636675f4a6872b1d34fa7d1a7a
Review-Url: https://codereview.chromium.org/2371833007
Cr-Commit-Position: refs/heads/master@{#43755}