std: Remove ManualThreads spawn mode
mk: Fix NO_REBUILD so stdtest can be tested without re-bootstrapping
std: Remove ThreadPerTask spawn mode. Unimplemented
std: Remove PlatformThread spawn mode. Obsolete
std: Remove ExistingScheduler spawn mode. Unused
std: Remove CurrentScheduler spawn mode. Unused
std: Remove get_task function. Unused
std: Remove foreign_stack_size spawn option. Irrelevant to future FFI changes