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/API/Syscall.h

@@ -118,6 +118,7 @@ enum class NeedsBigProcessLock {
     S(listen, NeedsBigProcessLock::Yes)                     \
     S(lseek, NeedsBigProcessLock::Yes)                      \
     S(madvise, NeedsBigProcessLock::Yes)                    \
+    S(map_time_page, NeedsBigProcessLock::Yes)              \
     S(mkdir, NeedsBigProcessLock::Yes)                      \
     S(mknod, NeedsBigProcessLock::Yes)                      \
     S(mmap, NeedsBigProcessLock::Yes)                       \