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Fix ::printf/etc. due to mutex size conflict (#677)

When real multicore/lockign support was added to newlib, there was an opaque
field in FILE that was used as a mutex, but was only 4 bytes in size.  The
recursive mutexes on the RP2040 are 8 bytes.  This mismatch caused corruption
of the FILE structure and crashes of the system when ::printf/::puts/etc. were
run.

Adjust the lock field size in FILE to 8 bytes and rebuild the toolchain to
fix.