Convert tdesc's expedite_regs to a string vector

Convert tdesc's expedite_regs to a string vector

Right now the list of expedited registers is stored as an array of char *,
with a nullptr element at the end to signal its last element.

Convert expedite_regs to a std::vector of std::string so it is easier to
manage the elements and the storage is handled automatically.

Eventually we might want to convert all the target functions so they pass a
std::vector of std::string as well. Or maybe expose an interface that target can
use to add expedited registers on-by-one depending on the target description
discovery needs, as opposed to just a static list of char *.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Tom Tromey <tom@tromey.com>