Fix timing issues found in "Flash - clock and cache test"

ARMCC seemed to be inlining time_cpu_cycles() but with a different number of clock cycles in the loop, GCC worked fine.

TESTS/mbed_hal/flash/functional_tests/main.cpp

@@ -53,7 +53,7 @@ static void erase_range(flash_t *flash, uint32_t addr, uint32_t size)
     }
 }
 
-static int time_cpu_cycles(uint32_t cycles)
+__attribute__((noinline)) static int time_cpu_cycles(uint32_t cycles)
 {
     Timer timer;
     timer.start();