Revert "SuperSize: Remove --tool-prefix"

This reverts commit 0340b93d.

Reason for revert: Broke diagnose_bloat.py

Original change's description:
> SuperSize: Remove --tool-prefix
>
> The idea that toolchains are interchangeable didn't really turn out to
> be true. SuperSize always works better with LLVM tools, with the
> exception of disassembling, and it simplifies the code to not pass
> around tool_prefix.
>
> The tool_prefix can still be overridden in tests via the environment
> variable: SUPERSIZE_TOOL_PREFIX
>
> Bug: None
> Change-Id: I169a4fff7221e25c72a548109739387771d8c41a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3383921


> Reviewed-by: Samuel Huang <huangs@chromium.org>
> Commit-Queue: Andrew Grieve <agrieve@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#959303}

Bug: None
Change-Id: I807e791352de473bcad695ddc60761c38f4905c8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3390031
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#959486}