Revert "Reimplement newlib syscalls to be reentrant"

This reverts commit 5e56eb8c11580633127656f9ea0ef0e63fdad966.

I have a private branch with the start of multithreaded newlib/rtx
changes.  Reverting this change from working branch so that I can
prepare a release soon without multithreading support and return to
that project later.