random: don't special case clock usage on macOS

Summary:
clock_gettime(), CLOCK_MONOTONIC and CLOCK_REALTIME are all available for use on
macOS (now that we require macOS >=10.12). Use them rather than the deprecated
mach_timespec_t time API.

master:
2019-12-23T20:49:43Z Feeding 216 bytes of dynamic environment data into RNG
2019-12-23T20:50:43Z Feeding 216 bytes of dynamic environment data into RNG

this commit:
2019-12-23T20:32:41Z Feeding 232 bytes of dynamic environment data into RNG
2019-12-23T20:33:42Z Feeding 232 bytes of dynamic environment data into RNG

Note: ABC dropped support for MacOS < 10.12 in 0.21.3

This is a backport of Core PR17800

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8774