PE: Extract get_pehdr_offset() for reuse

The get_pehdr_offset() function helps skip the DOS stub in a PE file to
locate the start of the PE header. Since it can be used in multiple C
files, it is being moved to a header file.

Signed-off-by: Pingfan Liu <piliu@redhat.com>
Signed-off-by: Simon Horman <horms@kernel.org>