CPython says thread identifier is a "nonzero integer", so rp2 should use a
1-indexed core number rather than 0-indexed. This fixes the
thread/thread_ident1 test failure on rp2 port.
Unfortunately this may be a breaking change for rp2 code which makes a
hard-coded comparison of thread identifier to 0 or 1.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>