update to get threads working

[mjs513]

@KurtE - @facchinm

Changes made to:
llext_exports.c
main.cpp in core arduino directory
linker_script.ld

Details of changes are shown in #37

Test sketches if you want to try:
thread_test_sketches.zip

EDIT: Just tried building for nano 33 sense ble and getting error

  /home/zephyr-sdk-0.16.8/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/12.2.0/../../../../arm-zephyr-eabi/bin/ld.bfd: app/libapp.a(llext_exports.c.obj):(._llext_const_symbol.static.__llext_sym___cxa_pure_virtual_+0x4): undefined reference to `__cxa_pure_virtual'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.
FATAL ERROR: command exited with status 1: /usr/bin/cmake --build /home/my_new_zephyr_folder/ArduinoCore-zephyr/build

it is set up in exports.c as:

FORCE_EXPORT_SYM(__cxa_pure_virtual);

See #44 (comment). Forgot to update conf files for other boards.

[mjs513]

@facchinm - @KurtE

See you moved the cpp stuff to proj.conf - forgot that you can do it that way :)