Fix all clang warnings in vfwprintf.c.

1.) Include stdlib.h for explicit definitions of wcstombs() and friends.
2.) Add parens to 1<<a-b to 1<<(a-b) to suppress a precendence warning.
3.) Use &"asdf"[index] instead of "asdf"+index to fix -Wstring-plus-int.

No intended functionality change.

Change-Id: I1529d448659847644f5688ee89bd4fb0a9f3af31