Fix firmware size check with avr-libc 1:2.0.0+Atmel3.6.2-1.1 (Debian bullseye) (#12951)

Debian bullseye (testing at the moment, but seems close to release) has
avr-libc 1:2.0.0+Atmel3.6.2-1.1 with some changes taken from the
Atmel-distributed toolchain.  In particular, the <avr/io.h> header for
ATmega32A (avr/iom32a.h) now defines the FLASHEND constant as `0x7FFFU`,
and that `U` suffix breaks the firmware size check code, because the
shell arithmetic expansion that is used to calculate `MAX_SIZE` does not
support those C-specific suffixes.

As a workaround, add `-D__ASSEMBLER__` to the C preprocessor invocation
that is used to expand those macros; in this case avr/iom32a.h defines
`FLASHEND` without the `U` suffix, and everything works as it did before
with older avr-libc versions.

The exact same code is present in two places; they are both changed,
even though the code in `tmk_core/avr.mk` is actually never used for
ATmega32A (and the header for ATmega32U4 does not add that `U` suffix to
`FLASHEND` for some reason).

tmk_core/avr.mk

@@ -295,7 +295,7 @@ ifneq ($(strip $(BOOTLOADER)), qmk-dfu)
 endif
 	make -C lib/lufa/Bootloaders/DFU/ clean
 	$(QMK_BIN) generate-dfu-header --quiet --keyboard $(KEYBOARD) --output lib/lufa/Bootloaders/DFU/Keyboard.h
-	$(eval MAX_SIZE=$(shell n=`$(CC) -E -mmcu=$(MCU) $(CFLAGS) $(OPT_DEFS) tmk_core/common/avr/bootloader_size.c 2> /dev/null | sed -ne 's/\r//;/^#/n;/^AVR_SIZE:/,$${s/^AVR_SIZE: //;p;}'` && echo $$(($$n)) || echo 0))
+	$(eval MAX_SIZE=$(shell n=`$(CC) -E -mmcu=$(MCU) -D__ASSEMBLER__ $(CFLAGS) $(OPT_DEFS) tmk_core/common/avr/bootloader_size.c 2> /dev/null | sed -ne 's/\r//;/^#/n;/^AVR_SIZE:/,$${s/^AVR_SIZE: //;p;}'` && echo $$(($$n)) || echo 0))
 	$(eval PROGRAM_SIZE_KB=$(shell n=`expr $(MAX_SIZE) / 1024` && echo $$(($$n)) || echo 0))
 	$(eval BOOT_SECTION_SIZE_KB=$(shell n=`expr  $(BOOTLOADER_SIZE) / 1024` && echo $$(($$n)) || echo 0))
 	$(eval FLASH_SIZE_KB=$(shell n=`expr $(PROGRAM_SIZE_KB) + $(BOOT_SECTION_SIZE_KB)` && echo $$(($$n)) || echo 0))

tmk_core/rules.mk

@@ -458,7 +458,7 @@ ifeq ($(findstring avr-gcc,$(CC)),avr-gcc)
 SIZE_MARGIN = 1024
 
 check-size:
-	$(eval MAX_SIZE=$(shell n=`$(CC) -E -mmcu=$(MCU) $(CFLAGS) $(OPT_DEFS) tmk_core/common/avr/bootloader_size.c 2> /dev/null | sed -ne 's/\r//;/^#/n;/^AVR_SIZE:/,$${s/^AVR_SIZE: //;p;}'` && echo $$(($$n)) || echo 0))
+	$(eval MAX_SIZE=$(shell n=`$(CC) -E -mmcu=$(MCU) -D__ASSEMBLER__ $(CFLAGS) $(OPT_DEFS) tmk_core/common/avr/bootloader_size.c 2> /dev/null | sed -ne 's/\r//;/^#/n;/^AVR_SIZE:/,$${s/^AVR_SIZE: //;p;}'` && echo $$(($$n)) || echo 0))
 	$(eval CURRENT_SIZE=$(shell if [ -f $(BUILD_DIR)/$(TARGET).hex ]; then $(SIZE) --target=$(FORMAT) $(BUILD_DIR)/$(TARGET).hex | $(AWK) 'NR==2 {print $$4}'; else printf 0; fi))
 	$(eval FREE_SIZE=$(shell expr $(MAX_SIZE) - $(CURRENT_SIZE)))
 	$(eval OVER_SIZE=$(shell expr $(CURRENT_SIZE) - $(MAX_SIZE)))