s390/vdso: add minimal compat vdso

Add a small vdso for 31 bit compat application that provides
trampolines for calls to sigreturn,rt_sigreturn,syscall_restart.
This is requird for moving these syscalls away from the signal
frame to the vdso. Note that this patch effectively disables
CONFIG_COMPAT when using clang to compile the kernel. clang
doesn't support 31 bit mode.

We want to redirect sigreturn and restart_syscall to the vdso. However,
the kernel cannot parse the ELF vdso file, so we need to generate header
files which contain the offsets of the syscall instructions in the vdso
page.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>