fix: ifdef linux specific cpu count compat

Expand the '#ifdef __linux__' block in src/compat-cpu.h to all static
inline functions related to sysfs since they are only useful on Linux
and fail to build on some non-Linux platforms. This issue was reported
on QNX.

The corresponding unit tests have to be skipped on non-Linux platforms.

Thanks to Elad Lahav <e2lahav@gmail.com> for reporting this issue.

Change-Id: I17c88a9a2fb5b9be6cf5325234a18ff40788cd09
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>