handle mremap failure in realloc of mmap-serviced allocations

handle mremap failure in realloc of mmap-serviced allocations

mremap seems to always fail on nommu, and on some non-Linux
implementations of the Linux syscall API, it at least fails to
increase allocation size, and may fail to move (i.e. defragment) the
existing mapping when shrinking it too. instead of failing realloc or
leaving an over-sized allocation that may waste a large amount of
memory, fallback to malloc-memcpy-free if mremap fails.