compiler: add minsize attribute for -Oz

This matches the behavior of Clang, which uses optsize for -Os and adds
minsize for -Oz.

The code size change is all over the map, but using a hacked together
size comparison tool I've found that there is a slight reduction in
binary size overall (-1.6% with the tinygo smoke tests and -0.8% for the
drivers smoke test).

stacksize/stacksize.go

@@ -180,8 +180,8 @@ func CallGraph(f *elf.File, callsIndirectFunction []string) (map[string][]*CallN
 					// used for getting a function pointer
 					isCall = false
 				case elf.R_ARM_ABS32:
-					// used in the reset vector for pointers
-					isCall = false
+					// when compiling with -Oz (minsize), used for calling
+					isCall = true
 				default:
 					return nil, fmt.Errorf("unknown relocation: %s", relocType)
 				}