stand: remove CLANG_NO_IAS from zfsldr

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.

Note that in some cases there are small differences in the generated
output, so CLANG_NO_IAS should be removed only after testing (or after
finding no differences in the output).  For zfsldr I compared objdump
output between GNU as- and Clang IAS-built zfsldr and .text was
identical (changes were limited to the object's ELF headers and debug
info).

Sponsored by:	The FreeBSD Foundation

stand/i386/zfsboot/Makefile

@@ -78,6 +78,3 @@ zfsboot.out: ${BTXCRT} zfsboot.o sio.o drv.o cons.o
 SRCS=	zfsboot.c
 
 .include <bsd.prog.mk>
-
-# XXX: clang integrated-as doesn't grok .codeNN directives yet
-CFLAGS.zfsldr.S=	${CLANG_NO_IAS}