arm: Use /proc/cpuinfo on linux if getauxval is unavailable

On really old libc versions, getauxval isn't available. Fall back
on /proc/cpuinfo in those cases, just like we do on android too.

src/arm/cpu.c

@@ -158,7 +158,7 @@ COLD unsigned dav1d_get_cpu_flags_arm(void) {
     return flags;
 }
 
-#elif defined(__ANDROID__)
+#elif defined(__ANDROID__) || defined(__linux__)
 #include <ctype.h>
 #include <stdio.h>
 #include <string.h>