xen/efi: don't use -fshort-wchar

Specifying -fshort-wchar for just one object breaks LTO with the
following error message:

 ld.gold: fatal error: Failed to link module drivers/xen/efi.o:
 linking module flags 'wchar_size': IDs have conflicting values

Since efi.c doesn't actually use wchar_t, turn off the flag when
LTO is enabled.

Bug: 62093296
Bug: 67506682
Change-Id: I509c18677353add8e1ad04f99f6e42bdab7814e7
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>