SmallerC: generate smaller & faster code; bugfixes

Improvements:
- use registers more efficiently
- eliminate some register copies and stack manipulations
- promote left operands to right immediate operands
- improve simple assignments to auto/static vars
- use bltz, blez, bgtz, bgez
- change mul/divu (by powers of 2) to sll/srl/and
- use seb, seh for sign extension instead of sll/sra
- overall generate a bit tighter, faster and more readable asm code

Fixes:
- >>= should be unsigned when the left operand is unsigned int.
The right operand isn't important here.
- very subtle bug in e.g. void f(){int a; &a+1;/*wrong value of &a+1*/}

Other:
- include stdarg.c and skeleton.c in /share/example/Makefile

share/example/Makefile

@@ -1,5 +1,5 @@
 
-all:	ashello echo chello
+all:	ashello echo chello stdarg skeleton
 
 ashello: ashello.o
 	$(LD) ashello.o -o $@
@@ -10,5 +10,11 @@ chello: chello.o
 echo:   echo.o
 	$(LD) $@.o -o $@
 
+stdarg: stdarg.o
+	$(CC) stdarg.o -o $@
+
+skeleton: skeleton.o
+	$(CC) skeleton.o -o $@
+
 clean:
-	rm -f *.o ashello echo chello *.dis *~
+	rm -f *.o ashello echo chello stdarg skeleton *.dis *~