Relax flaky _SC_AVPHYS_PAGES check in sys_sysinfo_test

The number of available physical pages is not constant and can
potentially change between calls to get_avphys_pages and
sysconf(_SC_AVPHYS_PAGES) calls.

Relax the assert to check that sconf(_SC_AVPHYS_PAGES) <= phys_pages.

Bug: http://b/31502852
Test: build and run bionic-unit-tests
Change-Id: Ibf8873d2151d93239391d2638dfbf055b70cde3a