efi_loader: fix building aarch64 EFI binaries

While our EFI binaries execute without problems on EDK II they crash on
a Lenovo X13s. Let our binaries look more like what EDK II produces:

* move all writable data to a .data section
* align sections to 4 KiB boundaries (matching EFI page size)
* remove IMAGE_SCN_LNK_NRELOC_OVFL from .reloc section flags

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>