cros: Pass a few flags in gn that are passed by the compiler wrapper etc

Depends on https://chromium-review.googlesource.com/c/chromium/src/+/3440879

This adds these two flags in Chrome's gn build files:

* --rtlib=compiler-rt
* --unwindlib=libunwind

They are implicitly already added by either cros_sdk's clang compiler
wrapper, or because cros_sdk's clang being build with cmake files
that add them.

However, for lacros builds with chromium's toolchain, they need to be
added explicitly. It doesn't hurt to always explicitly pass them for
all chromeos builds (and it would've helped me with grepping), so add
them always.

Also, only in lacros builds, add a --target flag.

There are more flags (for security hardening, performance, etc) that are
missing, but with this a lacros x86_64 build of the 'chrome' target with

    lacros_use_chromium_toolchain = true

builds successfully (but I haven't tried to run it). (Arm builds don't
work yet, but this sets the right triple for arm and even arm64 builds
here too. But some other things are missing for those configs.)

No behavior change for builds that don't set
lacros_use_chromium_toolchain (and currently nothing sets that).

Bug: 1169197
Change-Id: I4cf17b718a72a2914f505723e0efceb37a27aa3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3441379


Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#968369}