cpu: ldscript memory attribute corrections

[jnohlgard]

Contribution description

Using attributes (rx) on only the ROM segment will make the linker use that segment for orphan sections with read-only or executable contents.
Having a correct default placement for read-only and executable sections makes it easier to write augmentation ldscripts for adding more sections, such as XFA (#7523), because the linker will then place any added sections in ROM.
Any writable sections which need relocating during boot still needs the > ram AT> rom specifications like it is done in the platform specific ldscripts.

Quote GNU ld manual https://sourceware.org/binutils/docs/ld/MEMORY.html

The attr string is an optional list of attributes that specify whether to use a particular memory region for an input section which is not explicitly mapped in the linker script. As described in SECTIONS, if you do not specify an output section for some input section, the linker will create an output section with the same name as the input section. If you define region attributes, the linker will use them to select the memory region for the output section that it creates.

Issues/PRs references

based on #9084
useful in #7523
https://sourceware.org/binutils/docs/ld/MEMORY.html

[kYc0o]

ACK. Though the extra sections in some ldscripts are not used, I agree the most important it's the (!rx) field.