linker: llext: avoid modifying current address in linker script snippet

The linker script snippet that places the llext_no_syscall_impl section
has the unfortunate side effect of overwriting the current linker
address. This can lead to unexpected behavior of the caller script.

Saving the current address before defining the section and restoring it
afterwards avoids this issue.

Fixes #81136.

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