cmake: add_llext_target: handle empty LLEXT_REMOVE_FLAGS

When LLEXT_REMOVE_FLAGS is empty, the regular expression that is
constructed to remove flags from the Zephyr flags would match any
string, resulting in all flags being removed. This is not the intended
behavior, so we need to handle this case by setting the regular
expression to a pattern that does not match anything.

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