Implement sysconf for Arm

Implement sysconf for Arm

- add support for using sysconf to get page size in _mallocr.c via
  HAVE_SYSCONF_PAGESIZE flag set in configure.host
- set flag in configure.host for arm and add a default sysconf implementation
  in libc/sys/arm that returns the page size
- the default implementation can be overridden outside newlib to allow a
  different page size to improve malloc on devices with a small footprint
  without needing to rebuild newlib
- this patch is based on a contribution from Torbjorn Svensson and
  Niklas Dahlquist (https://ecos.sourceware.org/ml/newlib/current/017616.html)