Add configuration to prevent selection of Windows native toolchains when using Ubuntu under Windows 10

Back out most of 34be3e7c and update README again.  Windows native tools cannot be used with Ubuntu under Windows 10 now.  For Cygwin, that support depends on the 'cygpath -w' tool to convert POSIX paths to Windows paths.  There is no corresponding tool for Ubuntu under Windows 10.

Fix typos

SMP: Move sharable function to common file as irq_cpu_locked().  Use irq_cpu_locked() in sched_unlock()

SMP: Fix a gap where we may try to make modifications to the task lists without being in a critical sections.  That permits concurrent access to the tasks lists and many subtle problems.  This fix just remains in the critical section throughout the operation (and possible until the task is restore in the event of a context switch).  Makes a big difference in stability