s390/vdso: remove unused ENTRY in linker scripts

When linking vdso64.so.dbg with ld.lld, there is a warning about not
finding _start for the starting address:

ld.lld: warning: cannot find entry symbol _start; not setting start address

Fix this by removing the unused ENTRY in both vdso linker scripts. See
commit e247172854a5 ("powerpc/vdso: Remove unused ENTRY in linker
scripts"), which solved the same problem for powerpc, for further details.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

s390/vdso: add .got.plt in vdso linker script

KCFLAGS="-mno-pic-data-is-text-relative" make leads to bfd assertion
error in s390_got_pointer():

LD arch/s390/kernel/vdso64/vdso64.so.dbg
ld: BFD version 2.35-18.fc33 assertion fail elf-s390-common.c:74

readelf -Wr vdso64_generic.o | grep GOT
0000000000000032 000000110000001a R_390_GOTENT 0000000000000000 _vdso_data + 2
(...)

Add .got.plt in linker script to avoid this.

Suggested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

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>