llext: move settings away from cmake/compiler/* files

[pillo79]

Is your enhancement proposal related to a problem? Please describe.

PR #69831 adds the llext-edk build target, and uses the same location for configuration as llext itself: the files in the cmake/compiler/* subdirectories.
As discussed in this comment and related thread, this location is arguably acceptable for llext (since it's used for the current build), but definitely not for llext-edk, where the external compiler might be very different from the one used by Zephyr.

Describe the solution you'd like

The refactor is simple, once we figure out a location and storage criteria that satisfies the following constraints:

• It makes sense to keep llext and llext-edk settings grouped together
• They should be separated from other parameters related to the current build of the main Zephyr code
• These settings are architecture- and compiler- specific; however:
  • the llext settings refer to the same compiler used by the Zephyr build
  • the llext-edk settings are exported in a set of build files, and as such may refer to a different compiler

Ideas are welcome!