random: Use getrandom (GRND_RANDOM) in FIPS mode.

* random/rndgetentropy.c (_gcry_rndgetentropy_gather_random): Use
  GRND_RANDOM in FIPS Mode

The SP800-90C (clarified in IG D.K.) requires the following when
different DRBGs are chained:

• the parent needs to be reseeded before generate operation
• the reseed & generate needs to be atomic

In RHEL, this is addressed by change in the kernel, that will do this
automatically, when the getentropy () is called with GRND_RANDOM flag.

• Signed-off-by: Jakub Jelen <jjelen@redhat.com>