When parsing the target.xml register file, if no architecture has

been specified yet (either by the user, or by one of the lldb
extensions like qHostInfo or qProcessInfo), and the target.xml
includes a <architecture> tag specifying x86_64, set the architecture
appropriately.

I'm not sure what we can expect to see in the <architecture> tag, so
I'm only doing this for x86_64 right now where I've seen "i386:x86_64"
used.  I've seen a target.xml from a jtag board that sends just "arm"
because it doesn't know more specifically what type of board it is
connected to...  

<rdar://problem/29908970> 

llvm-svn: 322339