mmu: Anonymous memory demand paging

Switch to demand paging for anonymous virtual memory.

  SPECjvm 2008, higher is better:

                          ops/m
  benchmark               master  paging  +/- %
  ---------               ------  ------  -----
  compiler.compiler       347.06  380.95   +9.8
  compiler.sunflow        132.17  135.72   +2.7
  compress                107.59  125.02  +16.2
  crypto.aes               41.92   43.18   +3.0
  crypto.rsa              217.27  217.87   +0.3
  crypto.signverify       201.62  203.19   +0.8
  derby                   184.81  184.73   +0.0
  mpegaudio                78.86   78.71   -0.2
  scimark.fft.large        35.91   29.83  -16.9
  scimark.lu.large         14.36   16.85  +17.3
  scimark.sor.large        26.16   25.92   -0.9
  scimark.sparse.large     37.64   41.28   +9.7
  scimark.fft.small       176.99  207.04  +17.0
  scimark.lu.small        222.76  239.96   +7.7
  scimark.sor.small       115.81  116.84   +0.9
  scimark.sparse.small    119.97  125.74   +4.8
  scimark.monte_carlo     141.17  153.22   +8.5
  serial                  100.48  100.51   +0.0
  sunflow                  79.54   77.25   -2.9
  xml.transform           217.30  221.09   +1.7
  xml.validation          349.88  369.10   +5.5

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tst-mmap.so: mincore() tests for demand paging

As suggested by Nadav, add tests for mincore() interraction with demand
paging.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>