change and rename gmp_string_asprintf to return an std::string

This was suggested by Simon during a code review of this package upstream.
The upside is that this makes the function's API more natural and C++.
The downside is an extra malloc, which might be the reason why we went
for using a unique_xmalloc_ptr in the first place. Since this function
is not expected to be called frequently, the API improvement might be
worth the performance impact.

gdb/ChangeLog:

        * gmp-utils.h (gmp_string_printf): Rename from gmp_string_asprintf.
        Change return type to std::string. Update all callers.
        * gmp-utils.c (gmp_string_printf): Likewise.

gdb/gdbtypes.c

@@ -4927,7 +4927,7 @@ static void
 print_fixed_point_type_info (struct type *type, int spaces)
 {
   printfi_filtered (spaces + 2, "scaling factor: %s\n",
-		    fixed_point_scaling_factor (type).str ().get ());
+		    fixed_point_scaling_factor (type).str ().c_str ());
 }
 
 static struct obstack dont_print_type_obstack;