This fixes tests in PUGIXML_NO_XPATH mode on SPARC64 (#48).

SPARC does not allow unaligned accesses - e.g. you can't read an unaligned int.
Normally pugixml does not perform unaligned integer/pointer accesses, but page
heap can allocate blocks that are not aligned so that we can detect a single-
byte read/write overrun.

Additionally, the hardcoded page size we're currently using is really system
specific - on SPARC the page size can be 8 Kb instead of 4 Kb so mprotect can
fail.