Doesn't compile on Cortex-M0

[axos88]

It seems the Cortex-M0 doesn't have LDREX and STREX opcodes, so atomics are not implemented, so std::sync is not compiled into the alloc crate.

Either we should use our own implementation of Arc (using disabling interrupts or using the FreeRTOS critical sections), or figure out some way to avoid using them (at least) on platforms without atomic support