Kernel+LibC: Allow clock_gettime() to run without syscalls

This patch adds a vDSO-like mechanism for exposing the current time as
an array of per-clock-source timestamps.

LibC's clock_gettime() calls sys$map_time_page() to map the kernel's
"time page" into the process address space (at a random address, ofc.)
This is only done on first call, and from then on the timestamps are
fetched from the time page.

This first patch only adds support for CLOCK_REALTIME, but eventually
we should be able to support all clock sources this way and get rid of
sys$clock_gettime() in the kernel entirely. :^)

Accesses are synchronized using two atomic integers that are incremented
at the start and finish of the kernel's time page update cycle.

Kernel/Process.h

@@ -412,6 +412,7 @@ public:
     KResultOr<FlatPtr> sys$anon_create(size_t, int options);
     KResultOr<FlatPtr> sys$statvfs(Userspace<const Syscall::SC_statvfs_params*> user_params);
     KResultOr<FlatPtr> sys$fstatvfs(int fd, statvfs* buf);
+    KResultOr<FlatPtr> sys$map_time_page();
 
     template<bool sockname, typename Params>
     int get_sock_or_peer_name(const Params&);