[libj6] Add thread wrapper class

This new class makes it easier for user programs to spawn threads. This
change also includes support for .hh files in modules, to differentiate
headers that are C++-only in system libraries.

src/libraries/j6/thread.cpp

@@ -0,0 +1,50 @@
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+#include <string.h>
+#include <j6/errors.h>
+#include <j6/flags.h>
+#include <j6/syscalls.h>
+#include <j6/thread.hh>
+
+extern j6_handle_t __handle_self;
+
+namespace j6 {
+
+static constexpr size_t stack_size = 0x10000;
+static constexpr size_t zeros_size = 0x10;
+
+thread::thread(thread::proc p, uintptr_t stack_top) :
+    m_stack {j6_handle_invalid},
+    m_thread {j6_handle_invalid},
+    m_stack_top {stack_top},
+    m_proc {p}
+{
+    uintptr_t stack_base = stack_top - stack_size;
+    m_status = j6_vma_create_map(&m_stack, stack_size, stack_base, j6_vm_flag_write);
+    if (m_status != j6_status_ok)
+        return;
+
+    m_stack_top -= zeros_size;
+    memset(reinterpret_cast<void*>(m_stack_top), 0, zeros_size);
+}
+
+j6_status_t
+thread::start()
+{
+    if (m_status != j6_status_ok)
+        return m_status;
+
+    if (m_thread != j6_handle_invalid)
+        return j6_err_invalid_arg;
+
+    m_status = j6_thread_create(&m_thread, __handle_self,
+        m_stack_top, reinterpret_cast<uintptr_t>(m_proc));
+
+    return m_status;
+}
+
+}
+
+#endif // __j6kernel