[vm] Migrate away from `native 'name'` syntax.

As part of deprecating support for native extensions we are also
migrating away from legacy VM-specific `native 'name'` syntax
towards metadata based encoding which does not require any special
syntax.

This CL is a step 1 in migration:

- introduces support for `@pragma('vm:external-name', 'name')`
which serves as a direct replacement for `native 'name'`;
- all core libraries and tests are migrated to use the annotation;

Once this CL lands and rolls we will edit internal and external embedders
to eliminate uses of the native keyword (step 2) and finally remove
support for native keyword across our parsers (step 3).

TEST=ci

Bug: https://github.com/dart-lang/sdk/issues/28791
Change-Id: Id6dea878db82dd4fd81149243c425b5c5dc6df86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/212461
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>