[AVR] Add -MMD flag to generate depencencies files for S targets

Author: facchinm

hardware/arduino/avr/platform.txt

@@ -23,7 +23,7 @@ compiler.c.cmd=avr-gcc
 compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sections -fdata-sections -MMD -flto -fno-fat-lto-objects
 compiler.c.elf.flags={compiler.warning_flags} -Os -flto -fuse-linker-plugin -Wl,--gc-sections
 compiler.c.elf.cmd=avr-gcc
-compiler.S.flags=-c -g -x assembler-with-cpp -flto
+compiler.S.flags=-c -g -x assembler-with-cpp -flto -MMD
 compiler.cpp.cmd=avr-g++
 compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics -MMD -flto
 compiler.ar.cmd=avr-gcc-ar