libc - Use sbrk() system call, remove brk()

libc - Use sbrk() system call, remove brk()

* Attempt to use the kernel sbrk() system call.  If it fails
  with EOPNOTSUPP, back-off to the older libc sbrk() emulation
  code which uses still-present but obsolete break() system call.

* Changes support both old and new kernels.  Older kernels do not
  implement the sbrk() system call, causing the call to return
  -1/EOPNOTSUPP.  Note that older kernels did not generate an
  ENOSYS/signal for sbrk().

* sbrk() cannot support negative increments.  Allow only 0 and positive
  increments.

* Fix MPSAFE issues (requires new kernels) while we are here.

* Remove brk() support entirely.  This was also emulated in older
  kernels using break().

* These changes are necessary in order to support mmap() hinting above
  RLIMIT_DATA's current value.  Older kernels only support hinting above
  the system maximum value (above the 32GB mark).  Several languages in
  dports really want to mmap() addresses below 4GB.