[lldb] [Process/NetBSD] Remove unnecessary register buffer abstraction

Remove most of the abstraction over ptrace() register operations,
as it has little value and introduces more code than it saves.
Instead, leave a single ptrace() wrapper method and call it directly
from ReadRegisterSet() and WriteRegisterSet() with correct PT_* request
and buffer.

Remove the remaining direct ReadGPR() and WriteGPR() invocations
with ReadRegisterSet() and WriteRegisterSet().

Cleanup suggested by Pavel Labath in D63545.

Differential Revision: https://reviews.llvm.org/D63594

llvm-svn: 363923