Implement swapcontext() and makecontext()

This change introduces support for Linux-style uc_context manipulation
that's fast and works well on all supported OSes and architectures. It
also integrates with the Cosmpolitan runtime which can show backtraces
comprised of multiple stacks and fibers. See the test and example code
for further details. This will be used by Mold once it's been vendored

test/libc/calls/getcontext_test.c

@@ -83,3 +83,19 @@ void SetGetContext(void) {
 BENCH(getcontext, bench) {
   EZBENCH2("get/setcontext", donothing, SetGetContext());
 }
+
+BENCH(swapcontext, bench) {
+  ucontext_t main, loop;
+  volatile bool ready = false;
+  getcontext(&main);
+  if (ready) {
+    for (;;) {
+      swapcontext(&main, &loop);
+      // kprintf("boom\n");
+    }
+  } else {
+    ready = true;
+    EZBENCH2("x2 swapcontext", donothing, swapcontext(&loop, &main));
+    // kprintf("dollar\n");
+  }
+}