RFR: 8234397: add OS uptime information to os::print_os_info output

[Baesken, Matthias]

Hello,   please review this small addition  to  os::print_os_info  .

Currently os::print_os_info outputs various interesting OS information,
The output is platforms dependent, on Linux currently the following information is printed :
distro, uname , some important libversions, some limits, load average, memory info, info about /proc/sys , container and virtualization details and steal ticks.
The OS uptime would be a helpful addition.


Bug/webrev :
https://bugs.openjdk.java.net/browse/JDK-8234397
http://cr.openjdk.java.net/~mbaesken/webrevs/8234397.1/


Thanks, Matthias