Switch the rest of our internal headers to #pragma once.

We've been using #pragma once for new internal files, but let's be more bold.

Bug: N/A
Test: builds
Change-Id: I7e2ee2730043bd884f9571cdbd8b524043030c07
diff --git a/linker/linker_debug.h b/linker/linker_debug.h
index 7ceab08..862ea12 100644
--- a/linker/linker_debug.h
+++ b/linker/linker_debug.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef _LINKER_DEBUG_H_
-#define _LINKER_DEBUG_H_
+#pragma once
 
 // You can increase the verbosity of debug traces by defining the LD_DEBUG
 // environment variable to a numeric value from 0 to 2 (corresponding to
@@ -102,5 +101,3 @@
 #define MARK(x) do {} while (0)
 
 #endif
-
-#endif /* _LINKER_DEBUG_H_ */