OpenBSD: disable indirect branch tracking (crystal-lang#13665)

OpenBSD 7.4+ enforces Indirect Branch Tracking but we're not yet
compatible in Crystal, so we disable the feature for the time being.

src/compiler/crystal/compiler.cr

@@ -500,6 +500,13 @@ module Crystal
       else
         link_flags = @link_flags || ""
         link_flags += " -rdynamic"
+
+        if program.has_flag?("openbsd")
+          # OpenBSD requires Indirect Branch Tracking by default, but we're not
+          # compatible (yet), so we disable it for now:
+          link_flags += " -Wl,-znobtcfi"
+        end
+
         {DEFAULT_LINKER, %(#{DEFAULT_LINKER} "${@}" -o #{Process.quote_posix(output_filename)} #{link_flags} #{program.lib_flags(@cross_compile)}), object_names}
       end
     end