stand: remove CLANG_NO_IAS from btx and gptboot

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).

PR:		205250, 233094
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit 02c4bf4)

stand/i386/btx/btx/Makefile

@@ -29,6 +29,3 @@ ORG=	0x9000
 LDFLAGS+=${LDFLAGS_BIN}
 
 .include <bsd.prog.mk>
-
-# XXX: clang integrated-as doesn't grok .codeNN directives yet
-CFLAGS.btx.S=		${CLANG_NO_IAS}

stand/i386/btx/btxldr/Makefile

@@ -17,6 +17,3 @@ ORG=${LOADER_ADDRESS}
 LDFLAGS+=${LDFLAGS_BIN}
 
 .include <bsd.prog.mk>
-
-# XXX: clang integrated-as doesn't grok .codeNN directives yet
-CFLAGS.btxldr.S=	${CLANG_NO_IAS}

stand/i386/gptboot/Makefile

@@ -63,6 +63,3 @@ gptboot.out: ${BTXCRT} gptboot.o sio.o crc32.o drv.o cons.o ${OPENCRYPTO_XTS}
 	${LD} ${LD_FLAGS} --defsym ORG=${ORG2} -T ${LDSCRIPT} -o ${.TARGET} ${.ALLSRC} ${LIBSA32}
 
 .include <bsd.prog.mk>
-
-# XXX: clang integrated-as doesn't grok .codeNN directives yet
-CFLAGS.gptldr.S=	${CLANG_NO_IAS}