Go commit https://github.com/golang/go/issues/23672 introduced a
whitelist ofr flags passed into gcc to prevent arbitrary code
execution (CVE-2018-6574). The x86 rngd code uses two CFLAGS
not on the whitelist. Add them to 'CGO_CFLAGS_ALLOW'.
Signed-off-by: Rolf Neugebauer <rolf.neugebauer@gmail.com>