checkasm: Don't use readtime for checkasm seed

On ARM, the readtime implementations are instructions that might
not always be allowed at runtime (depending on whether the kernel
has allowed user mode code to access the cycle counter registers).
In order to allow building checkasm with the option for benchmarking,
while still running on devices where benchmarking isn't possible,
don't use readtime anywhere unless --bench has been specified.

Use GetTickCount for the seed on windows, and gettimeofday on unix.

tests/checkasm/checkasm.c

@@ -30,7 +30,6 @@
 #include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
-#include <time.h>
 
 #include "src/cpu.h"
 
@@ -39,11 +38,22 @@
 #define COLOR_RED    FOREGROUND_RED
 #define COLOR_GREEN  FOREGROUND_GREEN
 #define COLOR_YELLOW (FOREGROUND_RED|FOREGROUND_GREEN)
+
+static unsigned get_seed(void) {
+    return GetTickCount();
+}
 #else
 #include <unistd.h>
+#include <sys/time.h>
 #define COLOR_RED    1
 #define COLOR_GREEN  2
 #define COLOR_YELLOW 3
+
+static unsigned get_seed(void) {
+    struct timeval tv;
+    gettimeofday(&tv, NULL);
+    return tv.tv_usec + tv.tv_sec * 1000000;
+}
 #endif
 
 /* List of tests to invoke */
@@ -411,11 +421,7 @@ static void print_cpu_name(void) {
 
 int main(int argc, char *argv[]) {
     (void)func_new, (void)func_ref;
-#ifdef readtime
-    unsigned int seed = readtime();
-#else
-    unsigned int seed = time(NULL);
-#endif
+    unsigned int seed = get_seed();
     int ret = 0;
 
     while (argc > 1) {