Update Arch Linux ARM compiler installation instructions

- Add copy-pasteable Arch Linux `arm-none-eabi-gcc` install line similar to Ubuntu example
- Add `arm-none-eabi-newlib` as a required package for Arch
- Reformat Ubuntu and Arch install code-blocks to catch the eye for the impatient

Arch Linux changed their packaging for [arm-none-eabi-gcc](https://www.archlinux.org/packages/community/x86_64/arm-none-eabi-gcc/) by creating [arm-none-eabi-newlib](https://www.archlinux.org/packages/community/any/arm-none-eabi-newlib/) as an optional package. Without it users will get errors about missing header files like:

```
In file included from asf4/samd51/include/samd51j19a.h:49,
                 from asf4/samd51/include/sam.h:38,
                 from ./mpconfigport.h:31,
                 from ../../py/mpconfig.h:45,
                 from ../../py/emitnx64.c:3:
/usr/lib/gcc/arm-none-eabi/9.1.0/include/stdint.h:9:16: fatal error: stdint.h: No such file or directory
    9 | # include_next <stdint.h>
      |                ^~~~~~~~~~
compilation terminated.
```

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