loader/arm64/linux: Account for COFF headers appearing at unexpected offsets

loader/arm64/linux: Account for COFF headers appearing at unexpected offsets

The way we load the Linux and PE/COFF image headers depends on a fixed
placement of the COFF header at offset 0x40 into the file. This is
a reasonable default, given that this is where Linux emits it today.
However, in order to comply with the PE/COFF spec, which allows this
header to appear anywhere in the file, let's ensure that we read the
header from where it actually appears in the file if it is not located
at offset 0x40.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>