Makefile-AVR: solve the .siminfo section problem properly.

After researching this issue for the third time, I finally found
a proper solution: one can't keep an entire section without re-
writing the entire link script, but one can keep individual
symbols. That's what we do now, so we can use --gc-sections when
linking with SimulAVR support.

The problem came up again because -flto drops unused symbols, too.

This commit changes binary size drastically (1654 bytes less), so
let's take a new performance measurement snapshot:

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  17616 bytes      123%       58%       28%       14%
     Data:   2175 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

  short-moves.gcode statistics:
  LED on occurences: 888.
  LED on time minimum: 218 clock cycles.
  LED on time maximum: 395 clock cycles.
  LED on time average: 249.051 clock cycles.

  smooth-curves.gcode statistics:
  LED on occurences: 23648.
  LED on time minimum: 237 clock cycles.
  LED on time maximum: 438 clock cycles.
  LED on time average: 272.216 clock cycles.

  triangle-odd.gcode statistics:
  LED on occurences: 1636.
  LED on time minimum: 237 clock cycles.
  LED on time maximum: 395 clock cycles.
  LED on time average: 262.572 clock cycles.

Makefile-AVR

@@ -136,14 +136,16 @@ CFLAGS += -Wa,-adhlns=$(@:.o=.al)
 #CFLAGS += -dM -E # To list all predefined macros into the .o file.
 
 LDFLAGS  = -Wl,--as-needed
+LDFLAGS += -Wl,--gc-sections
 
 ifneq ($(realpath ../simulavr/src/simulavr_info.h),)
   # Neccessary for simulavr support, doesn't hurt others.
   CFLAGS += -DSIMINFO
   LDFLAGS += -Wl,--section-start=.siminfo=0x900000
-else
-  # Doesn't work for simulavr, can't allow dead code removal.
-  LDFLAGS += -Wl,--gc-sections
+  LDFLAGS += -u siminfo_device
+  LDFLAGS += -u siminfo_cpufrequency
+  LDFLAGS += -u siminfo_serial_in
+  LDFLAGS += -u siminfo_serial_out
 endif
 
 LIBS  = -lm