Implement function alignment via attributes

which requires being able to emit an arbitrary number of NOP
instructions, which is also implemented here.  For x86 we
could emit other sequences but these are the easiest.

c67-gen.c

@@ -2035,6 +2035,16 @@ void gfunc_epilog(void)
     }
 }
 
+ST_FUNC void gen_fill_nops(int bytes)
+{
+    if ((bytes & 3))
+      tcc_error("alignment of code section not multiple of 4");
+    while (bytes > 0) {
+	C67_NOP(4);
+	bytes -= 4;
+    }
+}
+
 /* generate a jump to a label */
 int gjmp(int t)
 {