Reconcile assembly-only macros in <machine/cpu-features.h>

The change explicitly isolates the assembly-only macros in header
<machine/cpu-features.h> in order to prevent mis-inclusion in C/C++
source files.

Change-Id: I0258e87c5ac3fd24944fb227290ac3b9cac4bfba
diff --git a/libc/arch-arm/include/machine/cpu-features.h b/libc/arch-arm/include/machine/cpu-features.h
index 5af325b..902fe45 100644
--- a/libc/arch-arm/include/machine/cpu-features.h
+++ b/libc/arch-arm/include/machine/cpu-features.h
@@ -158,6 +158,7 @@
 
 
 /* Assembly-only macros */
+#ifdef __ASSEMBLY__
 
 /* define a handy PLD(address) macro since the cache preload
  * is an optional opcode
@@ -168,4 +169,6 @@
 #  define  PLD(reg,offset)    /* nothing */
 #endif
 
+#endif /* ! __ASSEMBLY__ */
+
 #endif /* _ARM_MACHINE_CPU_FEATURES_H */