Replace all occurrances of asm with __asm__ in the output headers

asm() conflicts with userland code compiled with -std=c99, the userland
libc should only use __asm__() instead. Therefore, this transformation
has to be applied to all exported headers.

This only changes arch-arm/asm/byteorder.h.

Change-Id: I1cf88c37201c3a91668d387293a18885c316d53c
diff --git a/libc/kernel/tools/defaults.py b/libc/kernel/tools/defaults.py
index b35f72b..ca7e6bb 100644
--- a/libc/kernel/tools/defaults.py
+++ b/libc/kernel/tools/defaults.py
@@ -43,6 +43,11 @@
     "x86": {"__i386__": "1"},
     }
 
+# Replace tokens in the output according to this mapping
+kernel_token_replacements = {
+    {"asm": "__asm__"},
+    }
+
 # this is the set of known static inline functions that we want to keep
 # in the final ARM headers. this is only used to keep optimized byteswapping
 # static functions and stuff like that.