Arduino.h and avr stdlib "conflicting declaration of C function"

# Context

I'm working to get a Visual Studio Code Devcontainer up and running with a Bazel build for [Arduboy](https://www.arduboy.com/). I've pushed my current (problematic - per this GH issue) effort to my [arduboy_bazel repo](https://github.com/electronjoe/arduboy_bazel/tree/7b72a607f3eaf6027f18171bdf4cbc2587421543).

If you wanted to play with it you could:

- Check out the repo
- Install Docker Desktop
- Open directory in VS Code and install the VS Code [Remote - Containers extension](https://code.visualstudio.com/docs/remote/containers-tutorial)
- Restart vs code?
- `F1` -> `Remote-Containers: Rebuild and open in Container`
- Wait for the container to build
- Open terminal in VS Code
- `bazel build //example:buttons --config=avr --verbose_failures --sandbox_debug`
- Or for verbose action output
- `bazel aquery //example:buttons --config=avr --verbose_failures --sandbox_debug`

Alternatively you can dev entirely in the cloud and skip the vs code install / extension install by using the (free for ~40 hours per month) `gitpod.io` in your browser, point it at my repo.

I'm having trouble getting past the following compiler error in building an example `Arduboy` demo I've copied into my repo (`buttons.cpp`) [see ref](https://github.com/electronjoe/arduboy_bazel/blob/main/example/buttons.cpp).

# Versions

- I'm installing all `avr tooling` & `stdlib` using the `github.com/arduino/toolchain-avr` repo branch `7.3.0-atmel3.6.1-arduino7` [See here](https://github.com/electronjoe/arduboy_bazel/blob/main/.devcontainer/Dockerfile#L25-L36)

```
vscode ➜ /workspaces/arduboy_bazel (main ✗) $ /home/vscode/toolchain-avr/objdir/bin/avr-g++ --version
avr-g++ (GCC) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```

- I'm using `github.com/arduino/ArduinoCore-avr` at tag `1.8.5` [See here](https://github.com/electronjoe/arduboy_bazel/blob/main/deps.bzl#L56-L63)

# Compilation Command

`bazel build //example:buttons --config=avr --verbose_failures --sandbox_debug`
> /home/vscode/toolchain-avr/objdir/bin/avr-g++ -MD -MF bazel-out/avr-fastbuild/bin/example/_objs/buttons/main.d '-frandom-seed=bazel-out/avr-fastbuild/bin/example/_objs/buttons/main.o' -iquote . -iquote bazel-out/avr-fastbuild/bin -iquote external/arduboy2 -iquote bazel-out/avr-fastbuild/bin/external/arduboy2 -iquote external/arduinocore_avr -iquote bazel-out/avr-fastbuild/bin/external/arduinocore_avr -iquote external/bazel_tools -iquote bazel-out/avr-fastbuild/bin/external/bazel_tools -isystem external/arduboy2/src -isystem bazel-out/avr-fastbuild/bin/external/arduboy2/src -isystem external/arduinocore_avr/cores/arduino -isystem bazel-out/avr-fastbuild/bin/external/arduinocore_avr/cores/arduino -isystem external/arduinocore_avr/variants/leonardo -isystem bazel-out/avr-fastbuild/bin/external/arduinocore_avr/variants/leonardo -isystem external/arduinocore_avr/libraries/EEPROM/src -isystem bazel-out/avr-fastbuild/bin/external/arduinocore_avr/libraries/EEPROM/src '-mmcu=atmega32u4' '-DF_CPU=16000000L' '-DARDUINO=10819L' -DARDUINO_AVR_LEONARDO -DARDUINO_ARCH_AVR '-DUSB_VID=0x2341' '-DUSB_PID=0x8036' '-DUSB_MANUFACTURER="Arduboy"' '-DUSB_PRODUCT="Arduboy_FX"' -g -Os -w '-std=gnu++11' -fpermissive -fno-exceptions -ffunction-sections -fdata-sections -fno-threadsafe-statics '-Wno-error=narrowing' -flto -w -x c++ -CC -c external/arduinocore_avr/cores/arduino/main.cpp -o bazel-out/avr-fastbuild/bin/example/_objs/buttons/main.o

# Error

```
In file included from external/arduinocore_avr/cores/arduino/main.cpp:20:0:
external/arduinocore_avr/cores/arduino/Arduino.h:132:5: error: conflicting declaration of C function 'int atexit(void (*)(...))'
int atexit(void (*func)()) __attribute__((weak));
^~~~~~
In file included from external/arduinocore_avr/cores/arduino/Arduino.h:23:0,
from external/arduinocore_avr/cores/arduino/main.cpp:20:
/home/vscode/toolchain-avr/objdir/avr/include/stdlib.h:685:12: note: previous declaration 'int atexit(void (*)())'
extern int atexit(void (*)(void));
^~~~~~
```

And many more with similar root cause - [Full errors](https://gist.github.com/electronjoe/879a6425284576b90b22fd8a0b105db7).

# Thoughts

**Are there conflicting function declarations for a number of avr `stdlib` functions, redefined in `arduinocore_avr's Arduino.h`?**

Is my compilation mistreating C / C++ headers in some way?

Do you see anything I'm doing wrong here?

Thanks for any assistance!

Reproduce the Bazel AVR build for //example:buttons using the command shown, then inspect cores/arduino/main.cpp, Arduino.h around line 132, and the AVR stdlib.h declaration at line 685. Compare the additional errors in the linked full-error gist and determine the intended compatible declarations; done means the example builds without these conflicting-declaration errors.

Written by the indexing model from the issue text.