s390/vdso: Move vdso symbol handling to separate header file

The vdso.h header file, which is included at many places, includes
generated header files. This can easily lead to recursive header file
inclusions if the vdso code is changed.

Therefore move the vdso symbol code, which requires the generated
header files, to a separate header file, and include it at the two
locations which require it.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>