eal: improve entropy for initial PRNG seed

eal: improve entropy for initial PRNG seed

Replace the use of rte_get_timer_cycles() with getentropy() for
seeding the pseudo-random number generator. getentropy() provides a
more truly random value.

getentropy() requires glibc 2.25 and Linux kernel 3.17. In case
getentropy() is not found at compile time, or the relevant syscall
fails in runtime, the rdseed machine instruction will be used as a
fallback.

rdseed is only available on x86 (Broadwell or later). In case it is
not present, rte_get_timer_cycles() will be used as a second fallback.

On non-Meson builds, getentropy() will not be used.

Suggested-by: Bruce Richardson <bruce.richardson@intel.com>
Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>