A few more small tweaks to arm core file handling.
Most importantly, have DoReadGPR/DoReadFPU/DoReadEXC return -1
to indicate failure if they're called.  Else these could override
the Error setting for the relevant thread state -- if the core file
didn't include a floating point thread state, for instance, these
functions would clear the Error setting for that register set and
lldb would display random bytes as those registers' contents.
<rdar://problem/13665075>