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This is the first CL of an artefact investigating precise stack map
support for garbage collection in Blink / V8 using experimental LLVM
features.

The bulk of this change is made up of two main components:
  - A parser for reading the .llvm_stackmap section (generated by LLVM)
    into a table for use by the GC.

  - A simple stack walking mechanism, which, once called, traverses the
    call stack bottom up frame by frame searching for a gc rootset in the
    root table.

This also fleshes out a few helper API functions (see gc/gc_api.h) which
can be used when calling into the gc library from user code.

The idea for this gc/ directory is to be compiled as a library. It is
then linked to user code which has be compiled with the relevent LLVM IR
passes for generating stack maps at pre-defined safepoints. The code for
which will be forthcoming in a separate CL.

Bug: 978892

Change-Id: Ia2cb40881460b6a084babab9986df6c5303b41f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768770
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691113}