Support loom in rtic-common and rtic-sync. Fix rtic-sync bug.

[datdenkikniet]

Adds --cfg loom support to rtic-common and rtic-sync.

Additionally, adds a test for an rtic-sync bug & fixes it (I cannot find the bug report itself here on github).

Perhaps splitting out the bug fix into its own PR (d8f8a15) is desirable. I can do that if we deem that it makes sense.

Fixes #780

A description of the path leading to the bug, which also proves that the fix actually solves the problem:

1. try_recv pushes into freeq.
2. Before popping wait_queue, an interrupt occurs.
3. send no. 1 eats up the freeq slot.
4. send no. 2 is called, pushes its link to wait_queue and goes to sleep (because freeq is now empty)
5. Control returns to try_recv, which pops wait_queue (from the newly waiting send call) without touching the free queue.
6. send is awoken at some point. Its link is popped, but the freeq is empty.