kbuild: Enable -fshort-wchar

EFI_LOADER really wants UTF-16 strings (ie. %ls and L"string" are 16bit
chars instead of 32bit chars).  But rather than enabling -fshort-wchar
conditionally if EFI_LOADER is enabled, it was deemed preferrable to
globally switch.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>