This CL cleans up the trap handler code on POSIX before making additions
for arm64 simulator support.
In particular,
  - it extends a comment about restoring the signal mask
    before restoring the "thread in wasm" flag, and fixes the code to
    actually implement that again;
  - it renames "SigUnmaskStack" to "UnmaskOobSignalScope", to make the
    intent clear, and it moves the signal masking code to the
    constructor of that class;
  - it replaces a call to "IsThreadInWasm" by just reading
    "g_thread_in_wasm_code" to make it more transparent what is
    happening (note that the next instruction will just write to that
    flag);
  - it replaces an if block by another early exit for consistency; and
    lastly
  - it avoids curly braces for single-line conditions, to increase
    readability and to match the rest of V8.

R=ahaas@chromium.org, mseaborn@chromium.org

Bug: v8:11955
Change-Id: I023381f8b8e4640e2b21ac617fe301ec9f130783
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3015562
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75697}