efi: Compile kernel.img with -fshort-wchar on all EFI targets

efi: Compile kernel.img with -fshort-wchar on all EFI targets

The stack check logs a console message on failure, and the EFI API expects
a NULL terminated UCS-2 string. In order to define a UCS-2 string literal,
kernel.img on amd64 and i386 EFI targets is built with -fshort-wchar.

Also compile kernel.img on other EFI targets with -fshort-wchar.

Fixes: 37ddd94 (kern/efi/init: Log a console error during a stack check failure)
Reported-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Chris Coulson <chris.coulson@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>