Kernel processes need their own mman region

[mikbras]

Currently kernel processes (created with posix_spawn) attempt to share the mman region. We attempt to release any mmap'd memory when a process exists, however we are unable to release sbrk memory (which is allocated from the opposite of the mman region). For this to work for many processes, each process will need its own mman region (or the single mman region could be carved up).

[salsal97]

Once this change goes in, libcxx tests can be updated to not use 1 gb of heap memory since there is no release of sbrk memory happening now