x86: Fix zefi.py generation to use SDK toolchain

With SDK 0.12.2 we have support to generation EFI binaries in binutils
which is needed by the zefi.py script.  Now that is there we can utilize
the SDK objcopy instead of assuming the host objcopy can do this (which
would only be the case on x86 linux host systems).

Fixes #27047

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>