Silence warning about loss of precision

Explicitly cast to uintptr_t before casting to a 32-bit value.  Because this
code path is meant to be used in a 32-bit address space, this truncation should
be safe.

Unwind-EHABI.h:25:12: error: cast from pointer to smaller type 'uint32_t' (aka 'unsigned int') loses information

llvm-svn: 228357