Move from asm to __asm by default

- GCC with -std=c99 warns about asm but likes __asm
_ armcc5 has __asm but not asm
diff --git a/library/aesni.c b/library/aesni.c
index ff6c71d..9982b3b 100644
--- a/library/aesni.c
+++ b/library/aesni.c
@@ -37,6 +37,10 @@
 
 #include <string.h>
 
+#ifndef asm
+#define asm __asm
+#endif
+
 #if defined(MBEDTLS_HAVE_X86_64)
 
 /*