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Instead of wrapping the memory functions in the link stage, rebuild
Newlib and enable retargetable locks.  Override the weak definitions
in the libc.a with our own, SDK based ones.

The wrapping utilized before catches app-level memory allocations
but misses allocations inside Newlib libc (like printf/etc.).

Each core needs its own _impure_ptr or else crashes like the one seen
in parallel printf_floats can happen.  Enable it in the toolchain
build and implement a simple swapper here.

FreeRTOS SMP doesn't support Newlib's dynamic reent which is needed
to allow save MT support.  Minor patch to FreeRTOS and update the
FreeRTOS variant.cpp and setup to support it.