Define a utility function.
Fix typo.
Fix a bug that rejected valid code.

8cc couldn't compile this code.

  int f()
  int g() { f; }

The result of expression f in g is a pointer to f. The expression
does nothing because that has no side effect and the result is ignored,
but that is still valid.
Add static local variable.

8cc did not support that feature until now.

Static local variable is basically a global variable whose scope
is block scope. In this patch, we create a global variable object
for a local static variable and add that to the local environment.