rp2/modmachine: Add mutual exclusion for machine.lightsleep().HEADmaster

There's no specified behaviour for what should happen if both CPUs call
`lightsleep()` together, but the latest changes could cause a permanent
hang due to a race in the timer cleanup code.  Add a flag to prevent hangs
if two threads accidentally lightsleep, at least.

This allows the new lightsleep test to pass on RPI_PICO and RPI_PICO2, and
even have much tighter time deltas.  However, the test still fails on
wireless boards where the lwIP tick wakes them up too frequently.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

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