[Core] Squeeze AVR some more with `-mrelax` and `-mcall-prologues` (#16269)

platforms/avr/platform.mk

@@ -18,6 +18,13 @@ COMPILEFLAGS += -ffunction-sections
 COMPILEFLAGS += -fdata-sections
 COMPILEFLAGS += -fpack-struct
 COMPILEFLAGS += -fshort-enums
+COMPILEFLAGS += -mcall-prologues
+
+# Linker relaxation is only possible if
+# link time optimizations are not enabled.
+ifeq ($(strip $(LTO_ENABLE)), no)
+	COMPILEFLAGS += -mrelax
+endif
 
 ASFLAGS += $(AVR_ASFLAGS)
 
@@ -28,7 +35,7 @@ CFLAGS += -fno-strict-aliasing
 CXXFLAGS += $(COMPILEFLAGS)
 CXXFLAGS += -fno-exceptions -std=c++11
 
-LDFLAGS +=-Wl,--gc-sections
+LDFLAGS += -Wl,--gc-sections
 
 OPT_DEFS += -DF_CPU=$(F_CPU)UL