tests/llext: Compute linker script option rather than using -Wl,-T

Use the same logic as the top-level Zephyr CMakeLists.txt does to
find the linker script option by using the first one of these
which is set:

 1. Global TOPT property
 2. Compiler's linker_script property
 3. -Wl,-T

This avoids toolchains which insert a default linker script
when none is provided on the command line using the -T option.

Signed-off-by: Keith Packard <keithp@keithp.com>