[clang] [llvm] [AVR] Handle flash RO data mapped to data space for newer devices (PR #146244)

[Ben Shi via llvm-commits]

benshi001 wrote:

> > I do not think this PR is necessary, since there is no bug, I have explained in #146537.
> 
> You are right, so this PR is not a bug fix, but just adding features and a minor optimization.

Currently I am focusing on bugfix and compatibility/substitutability with avr-gcc-7.3 (which is used by Arduino major version), and quite cautious about adding new features.

For the `mflmap` / `mrodata-in-ram"` / `__do_flmap_init` mentioned in your PR, I am not familiar with them. So I need more time to
1. Understand these features in newer AVR devices,
2. study how gcc and air-libc support them,
3. check if they affect stability of existing llvm-avr's functionalities.

What's more, I can only decide modifications to the AVR backend. If you want to add new clang options, you have to create a proposal at https://discourse.llvm.org, and let the clang reviewers to decide.

https://github.com/llvm/llvm-project/pull/146244