Fix wrong CPU count after CPU hot-plugging

This fix tries to address issues raised in #23768 where the CPU count
is not updated after cpu ho-plugging.

This fix follows the suggestion from #23768 and replace go's `runtime.NumCPU()`
with `sysconf(_SC_NPROCESSORS_ONLN)` so that correct CPU count could
be obtained even after CPU hot-plugging.

This fix is tested manually, as is suggested in #23768.

This fix fixes #23768.

The NumCPU() in Linux is based on @wmark 's implementation.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Upstream-commit: 8b2383f5c1108c590928c0e6c57f1f0aa0dd36f6
Component: engine

components/engine/pkg/sysinfo/numcpu.go

@@ -0,0 +1,12 @@
+// +build !linux
+
+package sysinfo
+
+import (
+	"runtime"
+)
+
+// NumCPU returns the number of CPUs
+func NumCPU() int {
+	return runtime.NumCPU()
+}