AVR: Add option for strict address space subset relations.

To date, each named address space is implemented as a subset
of all the other address spaces, which allows code like

   char read_char (const char *addr, bool in_flash)
   {
      return in_flash
        ? *(const __flash char*) addr  // Read from program memory.
        : *addr;                       // Read from RAM.
   }

even though __flash is not a subset of Generic.

This patch introduces new target option -mstrict-addr-space-subsets
which instructs avr_addr_space_subset_p to implement the address
space subset relations like they actually are, i.e.

   __memx > __flashx > __flash, __flash1, ..., __flash5
   __memx > Generic

gcc/
	* config/avr/avr.opt (-mstrict-addr-space-subsets): New
	target option.
	* config/avr/avr.cc (avr_addr_space_subset_p): Use it.
	* doc/invoke.texi (AVR Options): Document it.