Fix --host-only problem with 5.x runtime

Look for Interp program headers to find host. It also finds libc-2.27.so and libpthread-2.27.so.

Issue: dotnet#179

Author: mikem8361

src/Microsoft.FileFormats/ELF/ELFFile.cs

@@ -109,6 +109,21 @@ public ELFSection FindSectionByName(string name)
             return null;
         }
 
+        public ELFProgramSegment FindProgramSegment(ELFProgramHeaderType type)
+        {
+            if (Header.ProgramHeaderOffset > 0 && Header.ProgramHeaderEntrySize > 0 && Header.ProgramHeaderCount > 0)
+            {
+                foreach (ELFProgramSegment segment in Segments)
+                {
+                    if (segment.Header.Type == type)
+                    {
+                        return segment;
+                    }
+                }
+            }
+            return null;
+        }
+
         private IEnumerable<ELFProgramSegment> ReadSegments()
         {
             Header.IsProgramHeaderCountReasonable.CheckThrowing();

src/Microsoft.SymbolStore/KeyGenerators/ELFFileKeyGenerator.cs

@@ -67,7 +67,8 @@ public override IEnumerable<SymbolStoreKey> GetKeys(KeyTypeFlags flags)
                     }
                     if ((flags & KeyTypeFlags.HostKeys) != 0)
                     {
-                        if (_elfFile.Header.Type == ELFHeaderType.Executable)
+                        if (_elfFile.Header.Type == ELFHeaderType.Executable || 
+                            _elfFile.FindProgramSegment(ELFProgramHeaderType.Interp) != null)
                         {
                             // The host program as itself (usually dotnet)
                             yield return BuildKey(_path, IdentityPrefix, buildId);