Avoid invalid memory optimizations (MarlinFirmware#20389)

When building for AVR, merge-all-constants can incorrectly combine constants stored in flash with constants stored in RAM. These have different access requirements, leading to undefined behavior during execution.
Co-authored-by: ellensp <ellensp@hotmail.com>

platformio.ini

@@ -203,7 +203,7 @@ extra_scripts      =
   pre:buildroot/share/PlatformIO/scripts/common-dependencies.py
   pre:buildroot/share/PlatformIO/scripts/common-cxxflags.py
   post:buildroot/share/PlatformIO/scripts/common-dependencies-post.py
-build_flags        = -fmax-errors=5 -g -D__MARLIN_FIRMWARE__ -fmerge-all-constants
+build_flags        = -fmax-errors=5 -g -D__MARLIN_FIRMWARE__ -fmerge-constants
 lib_deps           =
 
 #