In preparing for adding trap-based bounds checking to Windows, this
change refactors the code to separate the platform-specific portions
from that which can be shared between platforms.

Internally, we've renamed `RegisterDefaultSignalHandler` to
`RegisterDefaultTrapHandler` to more accurately represent the
difference in terminology between Linux (signals) and Windows
(exceptions). The external API is left the same so as not to break
downstream clients.

This CL is primarily to make room for Windows support. Future CLs
will begin adding support for Windows.

This is a reincarnation of https://crrev.com/c/626558.

Bug: v8:6743
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Iaa8bfd68c14cd1d17933b12c24cb8dd5ee8a21d6
Reviewed-on: https://chromium-review.googlesource.com/998829
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53006}