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}