AVR: Fix interrupt bombing during a context switch

TCB_RESTORE macro has a problem when restoring Status Register
and returning from the function (in up_fullcontextrestore()) as non-atomic action.

If there is some frequently occurring interrupt, chances are that we will
enter the interrupt handler just before ret is called.
The handler may cause a context switch which, when unrolled, will
execute up_fullcontextrestore() function that employs TCB_RESTORE.
It will be interrupted again just before return, leaving part of context
switch content un-popped again, etc... Thus, chances are that the stack will
eventually blow.

Note that this is not some edge condition fix. This bug was discovered when
testing AVR with UART configured to work on 115200 baud rate.