Kernel: Add a way to specify which memory regions can make syscalls

This patch adds sys$msyscall() which is loosely based on an OpenBSD
mechanism for preventing syscalls from non-blessed memory regions.

It works similarly to pledge and unveil, you can call it as many
times as you like, and when you're finished, you call it with a null
pointer and it will stop accepting new regions from then on.

If a syscall later happens and doesn't originate from one of the
previously blessed regions, the kernel will simply crash the process.

Kernel/Syscalls/mmap.cpp

@@ -550,4 +550,22 @@ void* Process::sys$allocate_tls(size_t size)
     return m_master_tls_region.unsafe_ptr()->vaddr().as_ptr();
 }
 
+int Process::sys$msyscall(void* address)
+{
+    if (m_enforces_syscall_regions)
+        return -EPERM;
+
+    if (!address) {
+        m_enforces_syscall_regions = true;
+        return 0;
+    }
+
+    auto* region = find_region_containing(Range { VirtualAddress { address }, 1 });
+    if (!region)
+        return -EINVAL;
+
+    region->set_syscall_region(true);
+    return 0;
+}
+
 }