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

Author: facchinm

hardware/arduino/sam/platform.txt

@@ -23,7 +23,7 @@ compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sectio
 compiler.c.elf.cmd=arm-none-eabi-gcc
 compiler.c.elf.flags=-Os -Wl,--gc-sections
 compiler.S.cmd=arm-none-eabi-gcc
-compiler.S.flags=-c -g -x assembler-with-cpp
+compiler.S.flags=-c -g -x assembler-with-cpp -MMD
 compiler.cpp.cmd=arm-none-eabi-g++
 compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -std=gnu++11 -ffunction-sections -fdata-sections -nostdlib -fno-threadsafe-statics --param max-inline-insns-single=500 -fno-rtti -fno-exceptions -Dprintf=iprintf -MMD
 compiler.ar.cmd=arm-none-eabi-ar