scripts/gdb/symbols: make lx-symbols skip the s390 decompressor

When one starts QEMU with the -S flag and attaches GDB, the kernel is
not yet loaded, and the current instruction is an entry point to the
decompressor. In case the intention is to debug the early kernel boot,
and not the decompressor, e.g., put a breakpoint on some kernel
function and see all the invocations, one has to skip the decompressor.

There are many ways to do this, and so far people wrote private scripts
or memorized certain command sequences.

Make it work out of the box like this:

    $ gdb -ex 'target remote :6812' -ex 'source vmlinux-gdb.py' vmlinux
    Remote debugging using :6812
    0x0000000000010000 in ?? ()
    (gdb) lx-symbols
    loading vmlinux
    (gdb) x/i $pc
    => 0x3ffe0100000 <startup_continue>:    lghi    %r2,0

Implement this by reading the address of the jump_to_kernel() function
from the lowcore, and step until DAT is turned on.

Signed-off-by: Ilya Leoshkevich <[email protected]>
Acked-by: Jan Kiszka <[email protected]>
Tested-by: Alexander Gordeev <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexander Gordeev <[email protected]>

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