[Core] Use -mcall-prologues the save code size

From GCC Manual:

Functions prologues/epilogues are expanded as calls
to appropriate subroutines. Code size is smaller.

Small programs with few functions can increase in size,
but QMK benefits from this as it is rather large.

See: https://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_optflags

platforms/avr/platform.mk

@@ -18,6 +18,7 @@ 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.