Avoid getrandom or syscall(SYS_getrandom) in Android __ANDROID_API__ < 28

[raidenluikang]

Android Os older than Android 9.0 (API_LEVEL = 28) broken syscall(SYS_getrandom).

Please, avoid it when building with NDK. Add something like

....
#elif defined(__linux__) || defined(__GNU__) || defined(GNU_KFREEBSD)

//---------- ADD THESE CODE --------------
#if defined(SYS_getrandom) && ((!defined(__ANDROID__) || __ANDROID_API__ >= 28)
       // use getrandom
       return randombytes_linux_randombytes_getrandom(buf, n);
#else 
    // use  /dev/urandom 
     return randombytes_linux_randombytes_urandom(buf, n);
 #endif
 //---------------------------------------
 #elif defined(BSD) ....

[raidenluikang]

For details see briansmith/ring#852

[dsprenkels]

I feel like in this case, because of the inability to determine which version of linux the Android system will run on, should we (like ring) always fall back to /dev/urandom/ if getrandom fails? Cc @thomwiggers

[thomwiggers]

Android versions < SDK API level 30 are no longer allowed for submission to the Google Play store, so this seems like an update that would not help very many people.

[thomwiggers]

Relatedly, it seems that all supported Linux kernels have getrandom; of course, there might be some redhat box kicking around though.