stand: remove CLANG_NO_IAS from boot2

Many components under stand/ had CLANG_NO_IAS added when Clang's
Integrated Assembler (IAS) did not handle .codeNN directives.  Clang
gained support quite some time ago, and we can now build stand/ with
IAS.  In most cases IAS- and GNU as-assembled boot components were
identical, and CLANG_NO_IAS was already removed from other components.

Clang IAS produces different output for some components, including
boot2, so CLANG_NO_IAS was not previously removed for those.

In the case of boot2 the difference is that IAS produces a larger
encoding for one instruction (the testb at the beginning of read).

GNU as produces:

    2e    f6 06 b0 08       80

while IAS includes an address size override prefix (67) and produces:

    2e 67 f6 05 b3 08 00 00 80

This results in three fewer NOPs elsewhere in boot2 but no functional
change, so switch to IAS for boot2.

(We can separately pursue improved 16-bit IAS support with the LLVM
developers.)

Sponsored by:	The FreeBSD Foundation

stand/i386/boot2/Makefile

@@ -91,6 +91,3 @@ boot2.h: boot1.out
 	    REL1=`printf "%d" ${REL1}` > ${.TARGET}
 
 .include <bsd.prog.mk>
-
-# XXX: clang integrated-as doesn't grok .codeNN directives yet
-CFLAGS.boot1.S=		${CLANG_NO_IAS}