scylla-gdb.py: make get_text_range() more reliable
scylla-gdb.py: make get_text_range() more reliable

Currenly `get_text_range()` uses heuristics about which ELF section
actually contains the text for the main executable. It appears that this
fails from time-to-time and we have to adjust the heuristics.
We don't really have to guess however, a much better method of
determining the section hosting text is to find a vtable pointer and
locate the section it resides in. For this, we use the
`reactor::_backend` as a canary. When this is not available, we fall
back to the pre-existing heuristics.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20200225164719.114500-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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