gdb/testsuite: fix failure in gdb.python/py-send-packet.exp

gdb/testsuite: fix failure in gdb.python/py-send-packet.exp

While working on another patch I noticed that, when run on an AArch64
target, the test gdb.python/py-send-packet.exp was failing:

  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/tmp/build/gdb/testsuite/outputs/gdb.python/py-send-packet/py-send-packet.py",
  line 106, in run_auxv_send_packet_test
      assert string == expected_result
  AssertionError
  Error while executing Python code.
  (gdb) FAIL: gdb.python/py-send-packet.exp: call python run_auxv_send_packet_test function

The test uses 'maint packet ...' to send a packet to gdbserver, and
then captures the output in TCL.  This output is then passed through
to a Python function, which performs some actions using the Python
API, and compares the results from the Python API to the results
captured in TCL from 'maint packet ...'.

The problem is that the output captured in TCL contains lots of things
like '\x000', when this is passed through to Python the '\x' causes
this to be treated as an escape code, which isn't what we want - we
want the actual string "\x000".

So, in the TCL part of the test we were expanding '\x' to '\\x', this
seemed to work fine for my testing on x86-64.

However, on AArch64 what I see is that the results from 'maint packet
...' contain a literal '\' character followed by a literal 'x'
character.  When GDB prints this in the 'maint packet' output, GDB
escapes the '\' for us, thus we get '\\x' printed by 'maint packet'.

However, now our TCL test script kicks in and tries to "fix" the '\x',
this means we now have '\\\x', which isn't correct.

The problem is that in the TCL script we are too restrictive, we
expand '\x' to '\\x', but really, we should be expanding all '\'
characters, regardless of what follows them.  This is what this patch
does.

After this the gdb.python/py-send-packet.exp test passes on AArch64
for me.