Kernel: Start working on a syscall for logging performance events

This patch introduces sys$perf_event() with two event types:

- PERF_EVENT_MALLOC
- PERF_EVENT_FREE

After the first call to sys$perf_event(), a process will begin keeping
these events in a buffer. When the process dies, that buffer will be
written out to "perfcore" in the current directory unless that filename
is already taken.

This is probably not the best way to do this, but it's a start and will
make it possible to start doing memory allocation profiling. :^)

Kernel/Syscall.h

@@ -175,7 +175,8 @@ typedef u32 socklen_t;
     __ENUMERATE_SYSCALL(set_process_boost)          \
     __ENUMERATE_SYSCALL(chroot)                     \
     __ENUMERATE_SYSCALL(pledge)                     \
-    __ENUMERATE_SYSCALL(unveil)
+    __ENUMERATE_SYSCALL(unveil)                     \
+    __ENUMERATE_SYSCALL(perf_event)
 
 namespace Syscall {