target/xtensa: list cores in a text file

target/xtensa: list cores in a text file

Avoid that leftover files affect the build; instead, use the same
mechanism that was in place before the Meson transition of updating
a file from import_core.sh.  Starting with Meson 0.57, the file
can be easily read from the filesystem module, so do that instead
of using run_command.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>