[libclc] Add -fshort-wchar to windows build after f07988ff3ec8
We were *hacking* the bc file in prepare-builtins by remvoing wchar_size
attributes since 8aa3513c8224b0f4c1f4d0f405f0facd955bd7f5
Aftre prepare-builtin removal in f07988ff3ec8,
we start to see linking failures in
SYCL :: OptionalKernelFeatures/is_compatible/is_compatible_amdgcn.cpp
SYCL :: OptionalKernelFeatures/is_compatible/is_compatible_nvptx64.cpp
This add the option to build libclc with -fshort-wchar on windows.
We are no using wchar anyhow.
