libgloss: msp430: delete custom .S.o rule

This was added to specify -Wa,-gdwarf2 when compiling the assembly
files.  Considering the size of these .S files, it seems unnecessary
overkill and complicates the build system.  All but one of these
compile down to 6 or fewer lines, and those hardly need extra debug
for anyone who is poking msp430 assembly.  The 1 file that is larger
than that (crt_move_highdata.S) is still <15 insns.

This will also simplify merging it into the top-level Makefile.

libgloss/msp430/Makefile.in

@@ -96,10 +96,6 @@ CRT_OBJS = \
 
 all: $(CRT) $(SIM_BSP) $(LIB_NOSYS) $(LIB_CRT) copy_scripts_to_objdir
 
-# Override .S.o rule to pass assembler debugging flags
-.S.o:
-	$(CC) -Wa,-gdwarf2 -Wa,-I$(srcdir) $(CFLAGS_FOR_TARGET) $(INCLUDES) $(CFLAGS) -c $<
-
 $(SIM_BSP): $(SIM_OBJS)
 	$(AR) $(ARFLAGS) $@ $?
 	$(RANLIB) $@