unbundling dependencies (distro packaging)

[selfisekai]

port analogical solutions from chromium:

• tool for replacing normal gn build files with unbundle ones, for use in such environments
• shim headers

I'm working on the same issue in flutter engine as well (flutter/flutter#121106, flutter/buildroot#687) - should at least some of the replacement build files be in a repository shared with flutter? (some are specific to flutter engine, some are shared with dart)

dependency-independent:

• [x] #51612
• [x] #51614

dependency list:

• [ ] icu (~~https://chromium-review.googlesource.com/c/chromium/deps/icu/+/4307370~~ https://dart-review.googlesource.com/c/sdk/+/303241)
• [ ] protobuf
• [x] ~~tcmalloc~~ (no longer valid - https://github.com/dart-lang/sdk/commit/7b2cfdbc8cd74cda8096acdcee4a8e112cf0c3b8)
• [ ] zlib (~~https://chromium-review.googlesource.com/c/chromium/src/+/4414588~~ https://dart-review.googlesource.com/c/sdk/+/303241)