Split the argument list into build and runtime dependencies and sort
everything alphabetically.
Unpin icu, it works with icu70, which is the latest version at this
time.
Remove yasm, it's not needed in firefox either.
Spidermonkey depends on rustc, so we have the best chance at only
having a single LLVM in the build closure if we use the same LLVM that
rustc depends on.
The configurePlatforms mkDerivation parameter needs to be an empty list
to avoid bogus defaults from breaking cross compilation.
Document the parameter to prevent it from being cleaned up.
