Make signed overflow behave "correctly"

Fix for issue arduino#4511
In short, the documentation says that signed integer overflow causes the value to "roll over".  Actually, this results in undefined behavior, with potentially unpredictable results.  This patch adds a GCC option to treat signed integers the "right" way so that they roll over as the documentation claims.

Author: cousteaulecommandant

hardware/arduino/avr/platform.txt

@@ -20,14 +20,14 @@ compiler.warning_flags.all=-Wall -Wextra
 # Default "compiler.path" is correct, change only if you want to overidde the initial value
 compiler.path={runtime.tools.avr-gcc.path}/bin/
 compiler.c.cmd=avr-gcc
-compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sections -fdata-sections -MMD
+compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -fwrapv -ffunction-sections -fdata-sections -MMD
 # -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={compiler.warning_flags} -Os -Wl,--gc-sections
 compiler.c.elf.cmd=avr-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp
 compiler.cpp.cmd=avr-g++
-compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD
+compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fwrapv -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD
 compiler.ar.cmd=avr-ar
 compiler.ar.flags=rcs
 compiler.objcopy.cmd=avr-objcopy