[Bug tdep/27789] backtraces through unknown functions on m68k are broken

["jrmuizel at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27789

--- Comment #6 from Jeff Muizelaar <jrmuizel at gmail dot com> ---
It's possible to just load the attached core file with a gdb that targets
m68k-linux (like ubuntu's gdb-multiarch)

gdb -c core
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warning: Can't open file /root/a.out during file-backed mapping note processing

warning: Can't open file /usr/lib/m68k-linux-gnu/ld-2.31.so during file-backed
mapping note processing

warning: Can't open file /usr/lib/m68k-linux-gnu/libc-2.31.so during
file-backed mapping note processing
[New LWP 395]
Core was generated by `./a.out'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x800003be in ?? ()
(gdb) bt
#0  0x800003be in ?? ()
#1  0xeff50c44 in ?? ()
#2  0x800003da in ?? ()
#3  0x00000004 in ?? ()
#4  0xeff50c50 in ?? ()
#5  0x800003ec in ?? ()
#6  0x00000000 in ?? ()
(gdb) q

notice the garbage frames: 1, 3, 4 and 6

Applying the attached patch which does the same thing as
7618e12bcb8f15e83c8c6abc5216f520f09bdbf9 gives:

(gdb) bt
#0  0x800003be in ?? ()
#1  0x800003da in ?? ()
#2  0x800003ec in ?? ()
#3  0x80000408 in ?? ()
#4  0xc004a902 in ?? ()
#5  0x80000346 in ?? ()

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