use raw syscalls instead of libSystem wrappers

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on October 08, 2013 15:14:32_

The libSystem mach wrappers seem pretty thin and reentrant, so the plan is
to initially use those for the Mac port ( issue #58 ).

There is no static libc support so we'll initially rely on the shared
libSystem. This issue covers eventually switching to hand-rolled syscalls.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1291_