[debugserver] Fix sign comparison warning.

Summary:
Comparing m_page_size against kInvalidPageSize was resulting in
a warning about comparing integers with different signs. Since
kInvalidPageSize isn't used anywhere outside of MachVMMemory.cpp,
we can readily transform it into a static const vm_size_t with
the correct value to avoid the sign comparison warnings.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12519

llvm-svn: 246606