Remove -MMD complier flag so no null.d file is created

See microsoft/vscode-arduino#1206 and https://developer.arm.com/documentation/101754/0616/armclang-Reference/armclang-Command-line-Options/-MD---MMD no idea why we would want this flag anyway.

Author: mcribbs

Makefile

@@ -1,7 +1,7 @@
 # Compiler flags
 AARCH64_TOOLCHAIN_DIR = build/aarch64-unknown-linux-gnu
 ARMGNU = $(AARCH64_TOOLCHAIN_DIR)/bin/aarch64-unknown-linux-gnu
-CFLAGS = -Wall -Wextra -ffreestanding -mgeneral-regs-only -MMD -mcpu=$(CPU)
+CFLAGS = -Wall -Wextra -ffreestanding -mgeneral-regs-only -mcpu=$(CPU)
 ASMFLAGS = -MMD
 LDFLAGS = -nostdlib -nostartfiles