llext: move all flags to compiler specific cmake files

Minimize the amount of flags that are hardcoded in the llext.cmake
module by moving them to the compiler specific cmake files.

The llext.cmake module will now use the new LLEXT_REMOVE_FLAGS and
LLEXT_APPEND_FLAGS global variables to adjust the set of flags used
to compile the llext code. This will make it easier to add support
for new architectures and compilers.

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