[build] Allow NO_WERROR=1 to inhibit --fatal-warnings as well as -Werror

src/Makefile.housekeeping

 CFLAGS		+= -Wall -W -Wformat-nonliteral
 CFLAGS		+= -g
 CFLAGS		+= $(EXTRA_CFLAGS)
-ASFLAGS		+= --fatal-warnings
 ASFLAGS		+= $(EXTRA_ASFLAGS)
 LDFLAGS		+= $(EXTRA_LDFLAGS)
 
 #
 ifneq ($(NO_WERROR),1)
 CFLAGS		+= -Werror
+ASFLAGS		+= --fatal-warnings
 endif
 
 # compiler.h is needed for our linking and debugging system