Casting pid to ::pid_t when invoking syscall.

Summary:
syscalls involving pid/tid on 32 bit binaries are failing with
"Invalid argument" because the uint64_t arguments are too wide.

Reviewers: clayborg, ovyalov, sivachandra

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7963

llvm-svn: 230817