This CL refactors the existing trap handler code for Linux to allow a
cleaner extension to Windows.

1) The CL extracts platform-specific code into separate files, see
https://docs.google.com/document/d/1HCgKIpdjy_CEodTLvZ5VuykDI6gGTHrTtau2j0zwm28.
Specifically this means:
* Move posix-specific API functions from v8.h to v8-wasm-trap-handler-posix.h.
  Deprecate the existing TryHandleSignal API function.
* Move posix-specific function declarations from trap-handler-internal.h to
  handler-inside-posix.h
* Move posix-specific function definitions from handler-shared.cc to
  handler-outside-posix.cc

2) The CL changes filenames from *-linux.* to *-posix.*. I expect that
most of the implementation for MacOS will be the same as for Linux.

Bug: v8:6743
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I4bb7f199564a2f01042084d15a82311d11a93c7b
Reviewed-on: https://chromium-review.googlesource.com/c/1280324
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57028}