Refactor avoid-default-panic to not rely on the linker script

[knoellle]

#812 introduced a feature to remove the default panicking behavior from for defmt::panic in case the user provides their own panic handler.
I don't know why this was implemented by modifying the linker script instead of just cfging out the whole function.

Not using the linker script for this also improves compatibility with macOS, see #1029 (comment)

Details

• defmt::panic logs its error message, then calls _defmt_panic to do the actual panicking.
• Previously, _defmt_panic wasn't defined anywhere but instead there was __defmt_default_panic.
• These two symbols were then combined in the linker script unless the avoid-default-panic feature was enabled.
• The new behavior just defines _defmt_panic IFF the feature is not enabled

[knoellle]

Damn, didn't think about the duplicate definitions.
That makes it a breaking change because users who were previously overriding the defmt panic handler are now required to use the avoid-default-panic feature to even compile.

The PROVIDES line in the linker script was effectively doing weak linking, which is supported by rust but hasn't been stabilized for over a decade now. rust-lang/rust#29603

Not sure if there is another way to achieve the same result without using either a linker script or an unstable feature.