[AVR] Add -g to elf.flags to allow inclusion of debug symbols in elf

Fixes #5539

Author: facchinm

hardware/arduino/avr/platform.txt

@@ -21,7 +21,7 @@ compiler.warning_flags.all=-Wall -Wextra
 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 -flto -fno-fat-lto-objects
-compiler.c.elf.flags={compiler.warning_flags} -Os -flto -fuse-linker-plugin -Wl,--gc-sections
+compiler.c.elf.flags={compiler.warning_flags} -Os -g -flto -fuse-linker-plugin -Wl,--gc-sections
 compiler.c.elf.cmd=avr-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp -flto -MMD
 compiler.cpp.cmd=avr-g++