efi_loader: Fix loaded image alignment

We are ignoring the alignment communicated via the PE/COFF header.
Starting 5.10 the Linux kernel will loudly complain about it. For more
details look at [1] (in linux kernel).

So add a function that can allocate aligned EFI memory and use it for our
relocated loaded image.

[1] c32ac11da3f83 ("efi/libstub: arm64: Double check image alignment at entry")

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Vincent Stehlé <vincent.stehle@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>