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>

gdbserver/tdesc.h

@@ -40,9 +40,9 @@ struct target_desc final : tdesc_element
   std::vector<tdesc_feature_up> features;
 
 #ifndef IN_PROCESS_AGENT
-  /* An array of register names.  These are the "expedite" registers:
+  /* A vector of register names.  These are the "expedite" registers:
      registers whose values are sent along with stop replies.  */
-  const char **expedite_regs = NULL;
+  std::vector<std::string> expedite_regs;
 
   /* Defines what to return when looking for the "target.xml" file in
      response to qXfer:features:read.  Its contents can either be