cmake: enable -mcall-prologues

Reduces flash memory usage by ~300B

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

@@ -127,6 +127,9 @@ if(CMAKE_CROSSCOMPILING)
   # split and gc sections
   add_compile_options(-ffunction-sections -fdata-sections)
   add_link_options(-ffunction-sections -fdata-sections -Wl,--gc-sections)
+
+  # Call Prologues/Epilogues to reduce code size
+  add_compile_options(-mcall-prologues)
 else()
   if(CMAKE_BUILD_TYPE STREQUAL "Debug")
     add_compile_options(-O0)