cmake: enable -mcall-prologues

Reduces flash memory usage by ~5KB

The CMake build outputs an ASM file for the firmware image  and is human readable.
To see the impacted functions, look for these routines:

__prologue_saves__
__epilogue_restores__

They should come in pairs.

For more info about this option, see:
https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc/AVR-Options.html#AVR-Options

CMakeLists.txt

@@ -341,6 +341,9 @@ if(CMAKE_CROSSCOMPILING)
   add_compile_options(-flto -fno-fat-lto-objects)
   add_link_options(-flto)
 
+  # Call Prologues/Epilogues to reduce code size
+  add_compile_options(-mcall-prologues)
+
   # Create this target before we apply the GC options
   add_library(avr_core STATIC ${AVR_SOURCES})
   set_reproducible_target(avr_core)