[PATCH] [gdb/tdep] Eliminate ostringstream use from arc_check_tdesc_feature

[Tom de Vries <tdevries@suse.de>]

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'.
...

[1] https://sourceware.org/pipermail/gdb-patches/2026-March/226117.html
---
 gdb/arc-tdep.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/gdb/arc-tdep.c b/gdb/arc-tdep.c
index ff283014e52..4936a5c8fbb 100644
--- a/gdb/arc-tdep.c
+++ b/gdb/arc-tdep.c
@@ -44,7 +44,6 @@
 
 /* Standard headers.  */
 #include <algorithm>
-#include <sstream>
 
 /* The frame unwind cache for ARC.  */
 
@@ -2100,16 +2099,16 @@ arc_check_tdesc_feature (struct tdesc_arch_data *tdesc_data,
 
       if (!found && reg.required_p)
 	{
-	  std::ostringstream reg_names;
+	  std::string reg_names;
 	  for (std::size_t i = 0; i < reg.names.size(); ++i)
 	    {
 	      if (i == 0)
-		reg_names << "'" << reg.names[0] << "'";
+		string_appendf (reg_names, "'%s'", reg.names[0]);
 	      else
-		reg_names << " or '" << reg.names[0] << "'";
+		string_appendf (reg_names, " or '%s'", reg.names[i]);
 	    }
 	  arc_print (_("Error: Cannot find required register(s) %s "
-		       "in feature '%s'.\n"), reg_names.str ().c_str (),
+		       "in feature '%s'.\n"), reg_names.c_str (),
 		       feature->name.c_str ());
 	  return false;
 	}

base-commit: d1cb5bd656a253cd69dbcd096f21f658cda697c7
-- 
2.51.0