llext-edk: merge target-specific file generation

This commit removes all CMake- or Makefile-specific variables from the
code and instead uses a single list for each type of flag. Where a
difference is needed between the two, a special marker is used.
These markers are replaced later in the target-specific area of the
write functions.

Escaping of problematic characters is also added to the write functions,
to ensure that quotes and backslashes in the flags are properly handled.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>