libroot/x86_64: rewrite system_time[_nsecs]() to C++

No functional change intended, just code deobfuscation.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
libroot/x86_64: implement get_cpuid() in user mode

cpuid is available in user mode as well and it doesn't look like there
are going to be any x86 platforms with significantly different CPUs anytime
soon.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>