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

src/cmd/smlrc/Makefile

@@ -3,7 +3,8 @@ include $(TOPSRC)/target.mk
 
 CFLAGS          = -Os -Wall -DMIPS -DNO_ANNOTATIONS -DNO_PREPROCESSOR \
                   -DNO_PPACK -D_RETROBSD -D__SMALLER_C_SCHAR__ \
-                  -D__SMALLER_C__ -D__SMALLER_C_32__ -DSTATIC
+                  -D__SMALLER_C__ -D__SMALLER_C_32__ -DSTATIC \
+                  -DNO_EXTRA_WARNS -DSYNTAX_STACK_MAX=2560
 
 # For cross compile
 #include $(TOPSRC)/cross.mk

src/cmd/smlrc/smlrc.c

@@ -3797,7 +3797,7 @@ int exprval(int* idx, int* ExprTypeSynPtr, int* ConstExpr)
         }
 
         if (SyntaxStack[*ExprTypeSynPtr][0] == tokUnsigned && tok == tokRShift)
-            stack[oldIdxRight + 1 - (oldSpRight - sp)][0] = tokURShift;
+          stack[oldIdxRight + 1 - (oldSpRight - sp)][0] = tokURShift;
 
         // ignore RightExprTypeSynPtr for the purpose of promotion/conversion of the result of <</>>
         RightExprTypeSynPtr = SymIntSynPtr;
@@ -4215,10 +4215,12 @@ int exprval(int* idx, int* ExprTypeSynPtr, int* ConstExpr)
         {
         case tokAssignDiv: t = tokAssignUDiv; break;
         case tokAssignMod: t = tokAssignUMod; break;
-        case tokAssignRSh: t = tokAssignURSh; break;
+        case tokAssignRSh:
+          if (SyntaxStack[*ExprTypeSynPtr][0] == tokUnsigned)
+            t = tokAssignURSh;
+          break;
         }
-        if (t != tok)
-          stack[oldIdxRight + 1 - (oldSpRight - sp)][0] = t;
+        stack[oldIdxRight + 1 - (oldSpRight - sp)][0] = t;
       }
 
       *ConstExpr = 0;