r304555, r304556, r304557, r304558, r304559, r304561, r304563, r304564,
  r304565, r304615, r304742, r304743, r304744, r304745, r304748, r304886,
  r304991, r305928, r305938, r305987, r306185:

Bring CloudABI support back in sync with HEAD.

- Add support for running 32-bit executables on amd64, armv6 and i386.

- As these new architectures require the use of the vDSO, merge back
  vDSO support for 64-bit executables running on amd64 and arm64 as
  well. This has the advantage that support for vDSO-less execution
  can be phased out when 11.0 becomes unsupported, as opposed to 11.x.

This change has been tested by running the cloudlibc unit tests on all
supported architectures, which seems to work fine.