SBHostOS: Fix a pointer-to-function to void-pointer cast

reinterpret_cast may not convert a pointer-to-function to a
void-pointer.  Take a detour through intptr_t and *then* convert to a
pointer-to-function.

This fixes a warning emitted by GCC.

Differential Revision: http://reviews.llvm.org/D4624

llvm-svn: 213692