arm64: fix arm64 Linux boot image header field sizes

The arm64 Linux boot protocol [1] describes the fields in the Image
header as being 64-bit little endian values.
So fix the endianess conversion to use 64-bit sized operations, for
both image_size and text_offset.
Also we use a local variable for the image_size to avoid both writing
to the header and also accessing it after we actually unmapped it.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arm64/booting.txt