[build] Treat warnings as errors in assembly files

Add --fatal-warnings to ASFLAGS; this is the equivalent of -Werror in
CFLAGS (which we have used since July 2007).

src/Makefile.housekeeping

 CFLAGS		+= -Wall -W -Wformat-nonliteral
 CFLAGS		+= -g
 CFLAGS		+= $(EXTRA_CFLAGS)
+ASFLAGS		+= --fatal-warnings
 ASFLAGS		+= $(EXTRA_ASFLAGS)
 LDFLAGS		+= $(EXTRA_LDFLAGS)