x86_64 firmware support: Use fixed size struct elements to support cross arch firmwares

As FSP binaries are released with a fixed architecture (x86_32), the header must *not* contain variable size fields like SIZE_T, UINTN, VOID* or UINTPTR_T. If compiled on x86_64 this would cause those structs to not match the ones used internally by FSP as the element size differs.

Please remove such elements and only use variables that have a fixed size, even on when compiled on x86_64.