Gdb protocol fixes

[bacam]

These fix a couple of problems that don't seem to affect gdb, but can confuse other tools that attempt to use the gdb protocol. One fixes reading S0 with p8 (gdb uses g), and the other is a duplicate response, which gdb appears to ignore.

[timsifive]

I have the exact same comment as for your other pull request. The code looks right. Did you run the test suite?

Thank you,
Tim

[bacam]

Running the tests picks up issue #84, because the duplicate response is effectively hiding the error. It's probably best not to merge this without sorting that out first.

[timsifive]

What errors are you seeing? I just ran the testsuite against your branch and everything worked.

[bacam]

I get failures for SimpleS1Test and WriteGprs:

(gdb) p/x $s1=0xe520e31011238d8
p/x $s1=0xe520e31011238d8
Could not write register "x9"; remote failure reply 'E03'

and

(gdb) p/x $x9=478198855569
p/x $x9=478198855569
Could not write register "x9"; remote failure reply 'E03'

respectively. This is for 64 bit; I think 32 bit would work. If I also apply #85 then it works for 64 bit.

[bacam]

I just checked this against the current head, and the debug test suite passes on both 32 bit and 64 bit.