[Mammoth] Move thread and process to the proc/ folder

lib/mammoth/proc/thread.cpp

@@ -0,0 +1,27 @@
+#include "proc/thread.h"
+
+#include <zcall.h>
+
+#include "mammoth/debug.h"
+#include "mammoth/init.h"
+
+namespace {
+
+extern "C" void thread_entry(Thread::Entry entry, void* arg1) {
+  entry(arg1);
+
+  (void)ZThreadExit();
+}
+
+}  // namespace
+
+Thread::Thread(Entry e, const void* arg1) {
+  check(ZThreadCreate(gSelfProcCap, &thread_cap_));
+  check(ZThreadStart(thread_cap_, reinterpret_cast<uint64_t>(thread_entry),
+                     reinterpret_cast<uint64_t>(e),
+                     reinterpret_cast<uint64_t>(arg1)));
+}
+
+glcr::ErrorCode Thread::Join() {
+  return static_cast<glcr::ErrorCode>(ZThreadWait(thread_cap_));
+}