[gdb/tdep] Eliminate ostringstream use from arc_check_tdesc_feature

We generally avoid C++ streams in GDB sources [1].

Remove an instance of std::ostringstream in arc_check_tdesc_feature.

While we're at it, fix an array index bug:
...
-		reg_names << " or '" << reg.names[0] << "'";
+		string_appendf (reg_names, " or '%s'", reg.names[i]);
...

Tested on x86_64-linux, using a trigger patch:
...
-      if (!found && reg.required_p)
+      if (true || (!found && reg.required_p))
...
and doing "maint selftest" and observing the output:
...
Running selftest unpack_field_as_long::ARC600.
Error: Cannot find required register(s) 'r0' in feature 'org.gnu.gdb.arc.core'.
Error: Cannot find required register(s) 'pc' in feature 'org.gnu.gdb.arc.aux'.
...

Approved-By: Simon Marchi <simon.marchi@efficios.com>

[1] https://sourceware.org/pipermail/gdb-patches/2026-March/226117.html