MachO: set sizeof result type to what embedded headers expect.

Embedded systems seem to have inherited Darwin's choise of "unsigned long" for
size_t (via a bunch of headers), so we should respect that.

rdar://problem/17872787

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214854 91177308-0d34-0410-b5e6-96231b3b80d8