Set EXTEXCLALL when enabling multi-core operation

[jannic]

Fixes #897

Also adds an on-target test to verify that atomics are working, and fixes initialization between on-target tests.

[thejpster]

Looks great.

Does the test fail if you don't enable the bit?

[jannic]

Does the test fail if you don't enable the bit?

Yes, and with a huge difference in the counter value:

(1/1) running `check_atomics`...
ERROR panicked at tests/multicore.rs:103:9:
assertion `left == right` failed
  left: 200000
 right: 100001

[thejpster]

Excellent!