Simplify NativeProcessProtocol::GetArchitecture/GetByteOrder

Summary:
These functions used to return bool to signify whether they were able to
retrieve the data. This is redundant because the ArchSpec and ByteOrder
already have their own "invalid" states, *and* because both of the
current implementations (linux, netbsd) can always provide a valid
result.

This allows us to simplify bits of the code handling these values.

Reviewers: eugene, krytarowski

Subscribers: javed.absar, lldb-commits

Differential Revision: https://reviews.llvm.org/D39733

llvm-svn: 317779