Move ObjectFile initialization out of SystemInitializerCommon

Summary:
For lldb-server, it is sufficient to parse only the native object file
format for its target OS (no other file can be loaded into a running
process). This moves the object file initialization code into specific
initializer classes: lldb-test and liblldb get all object files;
lldb-server gets only one of them. For this to work, I've needed to
create a special SystemInitializer for use in lldb-server, instead of it
calling directly into the common one.

This reduces the size of lldb-server by about 2%, which is not
earth-shattering, but it's an easy win, and it helps.

Reviewers: zturner, clayborg

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D47250

llvm-svn: 333182