Fix build failures with OPT = 0 due to inline functions (#19767)

Co-authored-by: Joel Challis <[email protected]>

platforms/timer.c

@@ -0,0 +1,8 @@
+// Copyright 2023 Sergey Vlasov (@sigprof)
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "timer.h"
+
+// Generate out-of-line copies for inline functions defined in timer.h.
+extern inline fast_timer_t timer_read_fast(void);
+extern inline fast_timer_t timer_elapsed_fast(fast_timer_t last);