fix(bench): separate vlibc-under-test TU from host harness TUs

benchmarks/bench_vlibc_under.c

@@ -0,0 +1,29 @@
+/*
+ * vlibc-under-test adapter for the bench_vlibc harness (todo 6 bench fix).
+ *
+ * This translation unit is compiled with vlibc's OWN headers (-I ../include)
+ * and is the ONLY TU in the benchmark that may include a vlibc header. The
+ * harness TU (bench_vlibc.c) must never include one: vlibc's self-contained
+ * <stdarg.h>/<stddef.h>/<limits.h>/<float.h> shadow GCC's internal headers,
+ * so any TU that mixes a vlibc header with the host <stdio.h>/<time.h>
+ * fails to compile (glibc's <stdio.h> needs __gnuc_va_list, which only the
+ * compiler's internal <stdarg.h> defines). The adapter isolates the
+ * vlibc-facing call here and exposes it to the host-header harness through
+ * its own declaration.
+ *
+ * Passing through the adapter also drops vlibc_version()'s
+ * __attribute__((const)) at the harness call site: the harness sees a plain
+ * external function, so the timed loop genuinely executes the call instead
+ * of being hoisted out by the optimizer.
+ */
+
+#include <vlibc.h>
+
+const char *
+bench_vlibc_version(void);
+
+const char *
+bench_vlibc_version(void)
+{
+    return vlibc_version();
+}