[NativeProcessLinux] Refactor PtraceWrapper

Summary:
This changes PtraceWrapper to return an Error, while the actual result is in an pointer parameter
(instead of the other way around). Also made a couple of PtraceWrapper arguments default to zero.
This arrangement makes a lot of the code much simpler.

Test Plan: Tests pass on linux. It compiles on android arm64/mips64.

Reviewers: chaoren, mohit.bhakkad

Subscribers: tberghammer, aemerson, lldb-commits

Differential Revision: http://reviews.llvm.org/D10808

llvm-svn: 241079