Added the notion of an system root for SDKs. This is a directory where all

libraries and headers exist. This can be specified using the platform select
function:

platform select --sysroot /Volumes/remote-root remote-macosx

Each platform subclass is free to interpret the sysroot as needed.

Expose the new SDK root directory through the SBDebugger class. 

Fixed an issue with the GDB remote protocol where unimplemented packets were
not being handled correctly.

llvm-svn: 133231