Implement getloadavg()

v2:
autoload PerfDataHelper functions
Keep loadavg in shared memory
Guard loadavg access by a mutex
Initialize loadavg to the current load

v3:
Shared memory version bump isn't needed if we are only extending it
Remove unused autoload
Mark inititalized flags as NO_COPY for correct behaviour in fork child

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

winsup/doc/posix.xml

@@ -1173,6 +1173,7 @@ also IEEE Std 1003.1-2008 (POSIX.1-2008).</para>
     getdtablesize
     getgrouplist
     getifaddrs
+    getloadavg
     getpagesize
     getpeereid
     getprogname