Adds support for conservative stack scanning on x64. The trampolines
saving callee-saved registers are compiled using clang (non-Windows)
and MASM (Windows). This is using the default toolchain for assembly
in Chromium/V8.

This differs from Oilpan in Chromium where x86 and x64 are compiled
using NASM [1]. V8 does not yet require this dependency and building
the trampolines natively avoids it. (NASM also requires separate
blocks for x64 Windows and non-Windows.) On non-x86/x64 platforms
Chromium also uses clang, so there's little benefit in keeping the
dependency.

The trampolines are tested when building with clang.

Other platforms follow in separate CLs.

[1] https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/platform/heap/asm/SaveRegisters_x86.asm

Change-Id: Ice2e23e44391aa94147abe75ee0b5afac458b8f8
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2124319
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66913}