Add errors on invalid/missing function return type (#8165)

GCC 10.x seems to have a knack for crashing when a function which is declared
to return a value does not.  Add a warning, present on all builds, when this
is the case.  For more info see #8160

Thanks to @hreintke and @mcspr for the tips.

platform.txt

@@ -21,11 +21,11 @@ runtime.tools.mkdir={runtime.platform.path}/tools/mkdir.py
 runtime.tools.cp={runtime.platform.path}/tools/cp.py
 runtime.tools.eboot={runtime.platform.path}/bootloaders/eboot/eboot.elf
 
-compiler.warning_flags=-w
-compiler.warning_flags.none=-w
-compiler.warning_flags.default=
-compiler.warning_flags.more=-Wall
-compiler.warning_flags.all=-Wall -Wextra
+compiler.warning_flags=-w -Werror=return-type
+compiler.warning_flags.none=-w -Werror=return-type
+compiler.warning_flags.default=-Werror=return-type
+compiler.warning_flags.more=-Wall -Werror=return-type
+compiler.warning_flags.all=-Wall -Wextra -Werror=return-type
 
 build.lwip_lib=-llwip_gcc
 build.lwip_include=lwip/include

tools/platformio-build.py

@@ -62,6 +62,7 @@ def scons_patched_match_splitext(path, suffixes=None):
         "-std=gnu17",
         "-Wpointer-arith",
         "-Wno-implicit-function-declaration",
+        "-Werror=return-type",
         "-Wl,-EL",
         "-fno-inline-functions",
         "-nostdlib"