[vm] Add support for real unboxed floating point fields in AOT

Non-nullable floating point fields (double, Float32x4, Float64x2)
are fully unboxed in their classes.

A bitmap for each class was added to the shared class table in order to keep
track of the pointers of the classes. Since all classes in Flutter Gallery
have less than 64 fields, the bitmap is represented by a 64 bit integer and
fields whose offset is more than 64 words are not unboxed.

The instance sizes and field offsets might change between target and host
in cross-compilation, since the number of words used to store unboxed fields
may differ.

dart-aot Xeon

  SplayLatency               -4.62%
  SplayHarderLatency         -4.17%
  NavierStokes               -2.20%
  Tracer                      8.72%
  ParticleSystemPaint         2.90%
  NBodySIMD                   8.35%
  NBody                      25.59%

With hack TFA to make doubles in Rect/Offset/Size classes in flutter non-nullable:

flutter arm-v8:

  gallery total size: -1%

  matrix_utils_transform_rect_perspective   -16.70% (less is better)
  matrix_utils_transform_rect_affine        -31.82% (less is better)
  matrix_utils_transform_point_perspective  -24.90% (less is better)
  matrix_utils_transform_point_affine)      -27.26% (less is better)
  rrect_contains_bench                      -4.719% (less is better)

Change-Id: I9ae09c9c3167d99f9efd071a92937aa51093fd1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/131824
Commit-Queue: Victor Agnez Lima <victoragnez@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Samir Jindel <sjindel@google.com>