Fixed compilation error when targetting Mega. Error is caused by arduino#2039 and this fix is just a workaround

Author: Federico Fissore

hardware/arduino/avr/platform.txt

@@ -17,7 +17,7 @@ compiler.c.cmd=avr-gcc
 compiler.c.flags=-c -g -Os -w -ffunction-sections -fdata-sections -MMD -flto
 # -w flag added to avoid printing a wrong warning http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59396
 # This is fixed in gcc 4.8.3 and will be removed as soon as we update the toolchain
-compiler.c.elf.flags=-Os -Wl,--gc-sections -w -flto
+compiler.c.elf.flags=-w -flto -Os -Wl,--gc-sections
 compiler.c.elf.cmd=avr-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp
 compiler.cpp.cmd=avr-g++