- 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)
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