sandbox: Fix building with LLVM

sandbox: Fix building with LLVM

- The macro __BIGGEST_ALIGNMENT__ is gcc-specific.  If it is not defined
  we'll just assume 16.  This is correct for at least the common cases
  and LLVM does not provide an equivalent macro.
- When linking U-Boot we're passing -T to the linker, and while gcc will
  just pass this along with LLVM we need to be specific.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>