[gdb/python] Make gdb.UnwindInfo.add_saved_register more robust

On arm-linux, until commit bbb12eb9c84 ("gdb/arm: Remove tpidruro register
from non-FreeBSD target descriptions") I ran into:
...
FAIL: gdb.base/inline-frame-cycle-unwind.exp: cycle at level 5: \
  backtrace when the unwind is broken at frame 5
...

What happens is the following:
- the TestUnwinder from inline-frame-cycle-unwind.py calls
  gdb.UnwindInfo.add_saved_register with reg == tpidruro and value
  "<unavailable>",
- pyuw_sniffer calls value->contents ().data () to access the value of the
  register, which throws an UNAVAILABLE_ERROR,
- this causes the TestUnwinder unwinder to fail, after which another unwinder
  succeeds and returns the correct frame, and
- the test-case fails because it's counting on the TestUnwinder to succeed and
  return an incorrect frame.

Fix this by checking for !value::entirely_available as well as
valued::optimized_out in unwind_infopy_add_saved_register.

Tested on x86_64-linux and arm-linux.

Approved-By: Andrew Burgess <aburgess@redhat.com>

PR python/31437
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31437

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