target/avr: Add instruction helpers

target/avr: Add instruction helpers

Add helpers for instructions that need to interact with QEMU. Also,
add stubs for unimplemented instructions. Instructions SPM and WDR
are left unimplemented because they require emulation of complex
peripherals. The implementation of instruction SLEEP is very limited
due to the lack of peripherals to generate wake interrupts. Memory
access instructions are implemented here because some address ranges
actually refer to CPU registers.

Signed-off-by: Michael Rolnik <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Signed-off-by: Aleksandar Markovic <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Aleksandar Markovic <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <20200705140315[email protected]>
[PMD: Replace cpu_physical_memory() API by address_space_ldst()
      API to fix running on big-endian host,
      reported and suggested by Peter Maydell]
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>