TryHandleSignal was originally limited by conditional compilation to only
platforms where the WebAssembly trap handler is supported. This caused build
problems, because not all the macros we needed were defined everywhere.

Instead, we make TryHandleSignal available on all POSIX platforms, but it
unconditionally returns false if the trap handler is not supported.

Bug: 
Change-Id: Iab4baf39b1708989edecc4ecfb51b926d8f7fe8d
Reviewed-on: https://chromium-review.googlesource.com/508838Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45468}