Add lldb-gdbserver support for Linux x86_64.

This change brings in lldb-gdbserver (llgs) specifically for Linux x86_64.
(More architectures coming soon).

Not every debugserver option is covered yet.  Currently
the lldb-gdbserver command line can start unattached,
start attached to a pid (process-name attach not supported yet),
or accept lldb attaching and launching a process or connecting
by process id.

The history of this large change can be found here:
https://github.com/tfiala/lldb/tree/dev-tfiala-native-protocol-linux-x86_64

Until mid/late April, I was not sharing the work and continued
to rebase it off of head (developed via id tfiala@google.com).  I switched over to
user todd.fiala@gmail.com in the middle, and once I went to github, I did
merges rather than rebasing so I could share with others.

llvm-svn: 212069

lldb/source/Utility/StringExtractor.cpp

@@ -418,6 +418,18 @@ StringExtractor::GetHexByteString (std::string &str)
     return str.size();
 }
 
+size_t
+StringExtractor::GetHexByteStringFixedLength (std::string &str, uint32_t nibble_length)
+{
+    str.clear();
+
+    uint32_t nibble_count = 0;
+    for (const char *pch = Peek(); (nibble_count < nibble_length) && (pch != nullptr); str.append(1, GetHexU8(0, false)), pch = Peek (), nibble_count += 2)
+    {}
+
+    return str.size();
+}
+
 size_t
 StringExtractor::GetHexByteStringTerminatedBy (std::string &str,
                                                char terminator)
@@ -428,6 +440,7 @@ StringExtractor::GetHexByteStringTerminatedBy (std::string &str,
         str.append(1, ch);
     if (Peek() && *Peek() == terminator)
         return str.size();
+
     str.clear();
     return str.size();
 }