llext: define add_llext_target() for llext compilation

This patch defines a generic function that encapsulates all the
architecture-specific machinery needed to compile llexts from source
files. Current tests are updated to use this function.

Output and source files must be specified using the OUTPUT and SOURCES
arguments. Only one source file is currently supported.

Arch-specific flags will be added automatically. The C_FLAGS argument
can be used to pass additional compiler flags to the compilation of
the source file.

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