x86: Pass -z execstack for EFI payload flags

To match how we link EFI executables elsewhere, and to silence a linker
warning, pass -z execstack here as well.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>