avr: pass the correct -mmcu flag to the linker

It does not appear to be necessary for these devices but might result in
more appropriate libraries to be linked in.

It is best to _not_ specify the exact MCU because otherwise a few other
settings (such as startfiles and some linker script configs) also get
set, which we do manually anyway and should not be interfered with.

I discovered this while working on support for the atmega1284.

targets/atmega328p.json

@@ -6,6 +6,9 @@
 	"cflags": [
 		"-mmcu=atmega328p"
 	],
+	"ldflags": [
+		"-mmcu=avr5"
+	],
 	"linkerscript": "src/device/avr/atmega328p.ld",
 	"extra-files": [
 		"targets/avr.S",

targets/digispark.json

@@ -7,6 +7,7 @@
 		"-mmcu=attiny85"
 	],
 	"ldflags": [
+		"-mmcu=avr25",
 		"-Wl,--defsym=_bootloader_size=2180",
 		"-Wl,--defsym=_stack_size=128"
 	],