Kernel: Add syscall performance event type

This allows tracing the syscalls made by a thread through the kernel's
performance event framework, which is similar in principle to strace.

Currently, this merely logs a stack backtrace to the current thread's
performance event buffer whenever a syscall is made, if profiling is
enabled. Future improvements could include tracing the arguments and
the return value, for example.

Kernel/UnixTypes.h

@@ -62,6 +62,7 @@ enum {
     PERF_EVENT_KMALLOC = 2048,
     PERF_EVENT_KFREE = 4096,
     PERF_EVENT_PAGE_FAULT = 8192,
+    PERF_EVENT_SYSCALL = 16384,
 };
 
 #define WNOHANG 1