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

SMP: Make checks for CPU lock set more robust.  There are certains conditions early in initialization on during interrupt handling where things need to be done a little differently.

SMP: There were certain conditions that we must avoid by preventing releasing the pending tasks while withn a critical section.  But this logic was incomplete; there was no logic to prevent other CPUs from adding new, running tasks while on CPU is in a critical section.

This commit corrects this.  This is matching logic in sched_addreadytorun to avoid starting new tasks within the critical section (unless the CPU is the holder of the lock).  The holder of the IRQ lock must be permitted to do whatever it needs to do.

SMP: Enforce this rule: Tasks which are normally restored when sched_unlock() is called must remain pending (1) if we are in a critical section, i.e., g_cpu_irqlock is locked , or (2) other CPUs still have pre-emption disabled, i.e., g_cpu_schedlock is locked.  In those cases, the release of the pending tasks must be deferred until those conditions are met.

Revert "SMP:  Fix a error introduced in 36b676ee and fully decouple disabling of pre-emption from critical sections."

This reverts commit 9aedf1bd.

Xtensa ESP32: Remove call to sched_lock()/unock() from inter-cpu interrupt logic.  Results in recursive call to sched_mergepending().

Revert "Xtensa SMP: Avoid a nasty situation in SMP by assuring that up_release_pending() is not re-entered."

This reverts commit 733a57b4.

This reverts commit fb146abe.

SMP:  Fix a error introduced in 36b676ee and fully decouple disabling of pre-emption from critical sections.