Cast pointers to uptr when calling internal_syscall

For Linux/x86-64, pointers passed to internal_syscall should be casted
to uptr first.  Otherwise, they won't be properly extended to 64-bit for
x32.
Patch by H.J. Lu


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209278 91177308-0d34-0410-b5e6-96231b3b80d8