cmake: add_llext_target: use toolchain-specific partial linking option

The partial linking option depends on the linker used in the build
process, and this flag is already defined by the Zephyr toolchains.
Use the $<TARGET_PROPERTY:linker,partial_linking> generator expression
to retrieve it instead of hardcoding the value for the GNU linker.

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