Add PAUSE to acquire and idleloop

With the mtrace QEMU, which now switches VCPUs when there's a PAUSE, this speeds up my lame test (usertests vmoverlap) by a factor of 7 and certainly makes xv6 feel much faster.

Add PAUSE to acquire and idleloop
c547c699 · Austin Clements · 426c0ecc · c547c699 · c547c699
--- a/kernel/idle.cc
+++ b/kernel/idle.cc
@@ -127,6 +127,7 @@ idleloop(void)
          exit();
      } while(worked);
      sti();
+      nop_pause();
    }
  }
 }

--- a/kernel/spinlock.cc
+++ b/kernel/spinlock.cc
@@ -301,8 +301,10 @@ acquire(struct spinlock *lk)
  locking(lk);
  retries = 0;
-  while(xchg32(&lk->locked, 1) != 0)
+  while(xchg32(&lk->locked, 1) != 0) {
    retries++;
+    nop_pause();
+  }
  locked(lk, retries);
 }