[gdb/testsuite] Use -std=c99 in gdb.base/nodebug.exp

With test-case gdb.base/nodebug.exp I run into:
...
gdb compile failed, gdb.base/nodebug.c: In function 'multf_noproto':
gdb.base/nodebug.c:63:1: warning: old-style function definition \
  [-Wold-style-definition]
   63 | multf_noproto (v1, v2)
      | ^~~~~~~~~~~~~
...

Fix this using -std=c99.

Tested on x86_64-linux.

PR testsuite/32756
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32756

gdb/testsuite/gdb.base/nodebug.exp

@@ -18,12 +18,17 @@
 
 standard_testfile .c
 
+set exec_opts {}
+if {[have_compile_flag -std=c99]} {
+    # Gcc 15 defaults to c23, which no longer supports unprototyped functions.
+    # Use a c dialect that does support this.
+    lappend exec_opts additional_flags=-std=c99
+}
+
 if {[test_compiler_info "xlc-*"]} {
     # By default, IBM'x xlc compiler doesn't add static variables into the symtab.
     # Use "-qstatsym" to do so.
-    set exec_opts additional_flags=-qstatsym
-} else {
-    set exec_opts ""
+    lappend exec_opts additional_flags=-qstatsym
 }
 
 if  { [gdb_compile $srcdir/$subdir/$srcfile $binfile executable $exec_opts] != "" } {