test failures on 32-bit architectures

[decathorpe]

I've recently been able to enable the futures test suite in Fedora RPM builds, however I have discovered to test failures on the following architectures:

• armv7-unknown-linux-gnueabihf (32-bit ARM)
• i686-unknown-linux-gnu (32-bit x86)

failures:
---- select_on_non_unpin_size stdout ----
thread 'select_on_non_unpin_size' panicked at 'assertion failed: `(left == right)`
  left: `32`,
 right: `16`', tests/async_await_macros.rs:229:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
---- select_size stdout ----
thread 'select_size' panicked at 'assertion failed: `(left == right)`
  left: `16`,
 right: `24`', tests/async_await_macros.rs:169:5
failures:
    select_on_non_unpin_size
    select_size

From what I can tell, this is caused by the hard-coded expected in-memory sizes being specific to 64-bit architectures.