stand: boot2: fix build with xtoolchain-llvm90

ufsread.c grows a dependency on __ashldi3 with llvm90. Grab ashldi3.c out of
compiler-rt rather than trying to link against libsa (for now).
-Wno-missing-prototypes is necessary to compile ashldi3.c standalone.

Reviewed by:	imp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D21291

stand/i386/boot2/Makefile

@@ -78,7 +78,12 @@ boot2.ldr:
 boot2.bin: boot2.out
 	${OBJCOPY} -S -O binary boot2.out ${.TARGET}
 
-boot2.out: ${BTXCRT} boot2.o sio.o
+# For __ashldi3
+.PATH: ${SRCTOP}/contrib/compiler-rt/lib/builtins
+CFLAGS.ashldi3.c=	-Wno-missing-prototypes
+CLEANFILES+=	ashldi3.o
+
+boot2.out: ${BTXCRT} boot2.o sio.o ashldi3.o
 	${LD} ${LD_FLAGS} -Ttext ${ORG2} -o ${.TARGET} ${.ALLSRC}
 
 SRCS=	boot2.c boot2.h