gdbstub: move fromhex/tohex routines to internals

gdbstub: move fromhex/tohex routines to internals

These will be needed from multiple places in the code. They are
declared as inline so move to the header and fix up to modern coding
style.

The only other place that messes with hex stuff at the moment is the
URI handling in utils but that would be more code churn so leave for
now.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230302190846.2593720-9-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-9-richard.henderson@linaro.org>