Support split debug symbols without build-id (gentoo-style) about orbit

[Chilledheart]

Errors showed by orbit in the same machine:

Could not load debug symbols for "/lib64/libc.so.6": Failed to find symbols for module "/lib64/libc.so.6" with build_id="":
- Unable to find local symbols for module "/lib64/libc.so.6": build id is empty.
- Not able to search for symbols on the instance.

Also: Could not load fallback symbols for "/lib64/libc.so.6": 
  * Could not find module file itself in cache: Unable to find object file in cache for module "/lib64/libc.so.6"
  * Could not find module file itself locally.
  * Could not search for module file itself on the instance.

Using readelf tool, it shows there is no build-id section:

➜  build-release git:(develop) readelf -n /lib64/libc.so.6

Displaying notes found in: .note.gnu.property
  Owner                Data size        Description
  GNU                  0x00000020       NT_GNU_PROPERTY_TYPE_0
      Properties: x86 feature used: x86, x87, XMM, YMM, ZMM, MASK
        x86 ISA used: x86-64-baseline, x86-64-v2, x86-64-v3, x86-64-v4

Displaying notes found in: .note.ABI-tag
  Owner                Data size        Description
  GNU                  0x00000010       NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 3.2.0

[Chilledheart]

It shows /usr/lib/debug is a hard coded path in lldb (debugger)'s source code lldb/source/Symbol/LocateSymbolFile.cpp

    // Add /usr/lib/debug directory.
    {
      FileSpec file_spec("/usr/lib/debug");
      FileSystem::Instance().Resolve(file_spec);
      debug_file_search_paths.AppendIfUnique(file_spec);
    }