random(4): depessimize arc4random

• __predict_false reseeding on entry as it is almost never true.
• don't blindly atomic_cmpset as on x86 it ends up dirtying the cacheline.

it almost ever succeeds per above

• fetch the timestamp prior to getting the cpu number

Reviewed by: cem
Approved by: secteam (delphij)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20242