Don't use .arch in aarch64 asm with Clang.

Clang (3.6, at least) doesn't like .arch when its internal as is used.
Instead, one has to pass -march=armv8-a+crypto on the command line.

Change-Id: Ifc5b57fbebd0eb53658481b0a0c111e808c81d93
Reviewed-on: https://boringssl-review.googlesource.com/4411
Reviewed-by: Adam Langley <agl@google.com>

crypto/aes/asm/aesv8-armx.pl

@@ -50,7 +50,11 @@ $code=<<___;
 #if __ARM_MAX_ARCH__>=7
 .text
 ___
-$code.=".arch	armv8-a+crypto\n"			if ($flavour =~ /64/);
+$code.=<<___ if ($flavour =~ /64/);
+#if !defined(__clang__)
+.arch  armv8-a+crypto
+#endif
+___
 $code.=".arch	armv7-a\n.fpu	neon\n.code	32\n"	if ($flavour !~ /64/);
 		#^^^^^^ this is done to simplify adoption by not depending
 		#	on latest binutils.

crypto/modes/asm/ghashv8-armx.pl

@@ -58,7 +58,11 @@ $code=<<___;
 
 .text
 ___
-$code.=".arch	armv8-a+crypto\n"	if ($flavour =~ /64/);
+$code.=<<___ if ($flavour =~ /64/);
+#if !defined(__clang__)
+.arch  armv8-a+crypto
+#endif
+___
 $code.=".fpu	neon\n.code	32\n"	if ($flavour !~ /64/);
 
 ################################################################################