[vm] Refactor StubCodeCompiler to be a real class.

Previously, StubCodeCompiler was just a set of static methods, all of
which take an assembler as their first arg. This makes it hard to pass
additional state to the ~160 stub macro defined stub generators.

This refactor makes StubCodeCompiler a real class, with assembler as a
field. So we can easily add new fields to the class later, to pass new
state without having to update every stub generator.

assembler is declared as a public field for a few reasons:
- There's one place where it needs to be accessed by a non-member
  function (in the ia32 file).
- If it's private, it has to be named assembler_, which would mean a lot
  more insignificant diffs.
- Non-member functions that take assembler would have to take assembler_,
  for consistency with the __ macro, which would be weird.

Change-Id: I142f0803a07c7839753188065c69c334d4d1798a
TEST=CI
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289924
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>