efi_selftest: compiler flags for efi_selftest_miniapp_exception.o

Add $(CFLAGS_EFI) and remove $(CFLAGS_NON_EFI) for
efi_selftest_miniapp_exception.o.

The removal is needed when compiling with LTO - this object file needs
to be compiled without -flto.

The adding is for consistency with other miniapps.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>