Because some processors are sensitive to the order of memory
accesses, add a few more variants of memory buffer backwards
copy which do sequential memory writes in the forward direction
inside of each sub-block of certain size. The most interesting
sizes of such sub-blocks are 32 and 64 bytes, because they match
the most frequently used CPU cache line sizes.

Example reports:

== ARM Cortex A7 ==
 C copy backwards                                     :    266.5 MB/s
 C copy backwards (32 byte blocks)                    :   1015.6 MB/s
 C copy backwards (64 byte blocks)                    :   1045.7 MB/s
 C copy                                               :   1033.3 MB/s

== ARM Cortex A15 ==
 C copy backwards                                     :   1438.5 MB/s
 C copy backwards (32 byte blocks)                    :   1497.5 MB/s
 C copy backwards (64 byte blocks)                    :   2643.2 MB/s
 C copy                                               :   2985.8 MB/s

This is expected to test the ability to do write combining for
scattered writes and detect any possible performance penalties.

Example reports:

== ARM Cortex A7 ==
 C fill                                               :   4011.5 MB/s
 C fill (shuffle within 16 byte blocks)               :   4112.2 MB/s (0.3%)
 C fill (shuffle within 32 byte blocks)               :    333.9 MB/s
 C fill (shuffle within 64 byte blocks)               :    336.6 MB/s

== ARM Cortex A15 ==
 C fill                                               :   6065.2 MB/s (0.4%)
 C fill (shuffle within 16 byte blocks)               :   2152.0 MB/s
 C fill (shuffle within 32 byte blocks)               :   2150.7 MB/s
 C fill (shuffle within 64 byte blocks)               :   2238.2 MB/s

== ARM Cortex A53 ==
 C fill                                               :   3080.8 MB/s (0.2%)
 C fill (shuffle within 16 byte blocks)               :   3080.7 MB/s
 C fill (shuffle within 32 byte blocks)               :   3079.2 MB/s
 C fill (shuffle within 64 byte blocks)               :   3080.4 MB/s

== Intel Atom N450 ==
 C fill                                               :   1554.9 MB/s
 C fill (shuffle within 16 byte blocks)               :   1554.5 MB/s
 C fill (shuffle within 32 byte blocks)               :   1553.9 MB/s
 C fill (shuffle within 64 byte blocks)               :   1554.4 MB/s

See https://github.com/ssvb/tinymembench/issues/7