ports/rp2: Use nano.specs for C++ modules.

Remove C++ stack unwinding and exception handling in all cases.

Set options as per https://github.com/raspberrypi/pico-sdk/blob/master/src/rp2_common/pico_cxx_options/CMakeLists.txt

Save around 10,880 bytes of RAM.

Signed-off-by: Phil Howard <phil@gadgetoid.com>

ports/rp2/CMakeLists.txt

@@ -374,6 +374,15 @@ target_compile_options(${MICROPY_TARGET} PRIVATE
 target_link_options(${MICROPY_TARGET} PRIVATE
     -Wl,--defsym=__micropy_c_heap_size__=${MICROPY_C_HEAP_SIZE}
 )
+# Do not include stack unwinding & exception handling for C++ user modules
+target_compile_definitions(usermod INTERFACE PICO_CXX_ENABLE_EXCEPTIONS=0)
+target_compile_options(usermod INTERFACE $<$<COMPILE_LANGUAGE:CXX>:
+    -fno-exceptions
+    -fno-unwind-tables
+    -fno-rtti
+    -fno-use-cxa-atexit
+>)
+target_link_options(usermod INTERFACE -specs=nano.specs)
 
 set_source_files_properties(
     ${PICO_SDK_PATH}/src/rp2_common/pico_double/double_math.c