s390/boot: Do not adjust GOT entries for undef weak sym

Since commit 778666df ("s390: compile relocatable kernel without
-fPIE") and commit 00cda11d ("s390: Compile kernel with -fPIC and
link with -no-pie") the kernel on s390x may have a Global Offset Table
(GOT) whose entries are adjusted for KASLR in kaslr_adjust_got().

The GOT may contain entries for undefined weak symbols that resolved to
zero. That is the resulting GOT entry value is zero. Adjusting those
entries unconditionally in kaslr_adjust_got() is wrong. Otherwise the
following sample code would erroneously assume foo to be defined, due to
the adjustment changing the zero-value to a non-zero one:

  extern int foo __attribute__((weak));
  if (*foo)
    /* foo is defined [or undefined and erroneously adjusted] */

The vmlinux build at commit 00cda11d ("s390: Compile kernel with
-fPIC and link with -no-pie") with defconfig actually had two GOT
entries for the undefined weak symbols __start_BTF and __stop_BTF:

$ objdump -tw vmlinux | grep -F "*UND*"
0000000000000000  w      *UND*  0000000000000000 __stop_BTF
0000000000000000  w      *UND*  0000000000000000 __start_BTF

$ readelf -rw vmlinux | grep -E "R_390_GOTENT +0{16}"
000000345760  2776a0000001a R_390_GOTENT      0000000000000000 __stop_BTF + 2
000000345766  2d5480000001a R_390_GOTENT      0000000000000000 __start_BTF + 2

The s390-specific vmlinux linker script sets the section start to
__START_KERNEL, which is currently defined as 0x100000 on s390x. Access
to lowcore is performed via a pointer of 0 and not a symbol in a section
starting at 0. The first 64K are reserved for the loader on s390x. Thus
it is safe to assume that __START_KERNEL will never be 0. As a result
there cannot be any defined symbols resolving to zero in the kernel.

Note that the first three GOT entries are reserved for the dynamic
loader on s390x. [1] In the kernel they are zero. Therefore no extra
handling is required to skip these.

Skip adjusting GOT entries with a value of zero in kaslr_adjust_got().

While at it update the comment when a GOT exists on s390x. Since commit
00cda11d ("s390: Compile kernel with -fPIC and link with -no-pie")
it no longer only exists when compiling with Clang, but also with GCC.

[1]: s390x ELF ABI, section "Global Offset Table",
     https://github.com/IBM/s390x-abi/releases

Fixes: 778666df

 ("s390: compile relocatable kernel without -fPIE")
Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>