Names with two leading underscores are reserved in any context of the c
standard, and thus must not be used. This ATmega platform used it however for
defining internal stuff. This commit fixes this.
To prevent reordering of accesses to the interrupt control register when link
time optimization (LTO) is enabled, memory barriers are needed. Without LTO
calls to the external functions irq_disable(), irq_restore(), irq_enable() and
irq_is_in() have the same affect as compiler barriers, as the compiler is unable
to prove that reordering of memory accesses is safe (from a single-threaded
point of view). With LTO the compiler can easily prove that reordering is safe
from a single-threaded point of view: Thus, the compiler may move memory
accesses wrapped in irq_disable(), irq_restore() across those calls.
The memory barriers will have no effect on non-LTO builds.
Citing the doc of irq_enable():
@return Previous value of status register. [...]
On atmega however the new value of the status register is returned, not the one
prior to enabling interrupts.
