Re-add deprecated _SPIFFS_xxx linker symbols

[earlephilhower]

In order to give user libs a change to update to the new symbols, re-add
the _SPIFFS_XX symbols to the linker file with a comment that they are
deprecated.

Also add back spiffs_hal_xxx functions, also marked as deprecated.

Fixes #6542

[earlephilhower]

Old user code can build now, and will emit deprecated warnings...

/home/earle/Arduino/sketch_sep13a/sketch_sep13a.ino: In function 'void setup()':
/home/earle/Arduino/sketch_sep13a/sketch_sep13a.ino:16:27: warning: '_SPIFFS_end' is deprecated (declared at /home/earle/Arduino/hardware/esp8266com/esp8266/cores/esp8266/spiffs_api.h:46) [-Wdeprecated-declarations]
   Serial.printf("%08x\n", _SPIFFS_end);
                           ^
/home/earle/Arduino/sketch_sep13a/sketch_sep13a.ino:17:3: warning: 'int32_t spiffs_hal_erase(uint32_t, uint32_t)' is deprecated (declared at /home/earle/Arduino/hardware/esp8266com/esp8266/cores/esp8266/spiffs_api.h:56) [-Wdeprecated-declarations]
   spiffs_hal_erase(1,1);
   ^
/home/earle/Arduino/sketch_sep13a/sketch_sep13a.ino:17:23: warning: 'int32_t spiffs_hal_erase(uint32_t, uint32_t)' is deprecated (declared at /home/earle/Arduino/hardware/esp8266com/esp8266/cores/esp8266/spiffs_api.h:56) [-Wdeprecated-declarations]
   spiffs_hal_erase(1,1);
                       ^