Port to FreeBSD

[Low-power]

Tested with FreeBSD 10 and 11.

Includes pull request #25 for correct building for FreeBSD i386.

Major changes/fixes:

• FreeBSD have their own runetype.h, renamed Darwin-specific runetype.h to mac-runetype.h just like mac-ctype.h, alone with associated identifiers, to avoid conflicts.
• Neither FreeBSD kernel nor C library provide *stat64, *statfs64 and fseeko64, avoid using them in such case.
• Implements getting executable file path using sysctl(3), in addition to the Linux-specific /proc/self/exe.
• Handles some differences in pthread-related macros between GNU and FreeBSD.
• qsort_r(3) has different prototype between GNU and FreeBSD, handled it accordingly.
• Function MachOLoader::loadSegments(const MachO &, intptr *, intptr *) contains a bug where it calls mmap(2) with MAP_ANON but 0 as fd, this is invalid, as kindly pointed out by FreeBSD kernel by a EINVAL.