scripts/decodecode: improve faulting line determination

There are cases where the IP pointer in a Code: line in an oops doesn't
point at the beginning of an instruction:

Code: 0f bd c2 e9 a0 cd b5 e4 48 0f bd c2 e9 97 cd b5 e4 0f 1f 80 00 00 00 00 \
	  e9 8b cd b5 e4 0f 1f 00 66 0f a3 d0 e9 7f cd b5 e4 0f 1f <80> 00 00 00 \
	  00 0f a3 d0 e9 70 cd b5 e4 48 0f a3 d0 e9 67 cd b5

  e9 7f cd b5 e4          jmp    0xffffffffe4b5cda8
  0f 1f 80 00 00 00 00    nopl   0x0(%rax)
	^^

and the current way of determining the faulting instruction line doesn't
work because disassembled instructions are counted from the IP byte to
the end and when that thing points in the middle, the trailing bytes can
be interpreted as different insns:

  Code starting with the faulting instruction
  ===========================================
     0:   80 00 00                addb   $0x0,(%rax)
     3:   00 00                   add    %al,(%rax)

whereas, this is part of

0f 1f 80 00 00 00 00    nopl   0x0(%rax)

     5:   0f a3 d0                bt     %edx,%eax
     ...

leading to:

  1d:   0f 1f 00                nopl   (%rax)
  20:   66 0f a3 d0             bt     %dx,%ax
  24:*  e9 7f cd b5 e4          jmp    0xffffffffe4b5cda8               <-- trapping instruction
  29:   0f 1f 80 00 00 00 00    nopl   0x0(%rax)
  30:   0f a3 d0                bt     %edx,%eax

which is the wrong faulting instruction.

Change the way the faulting line number is determined by matching the
opcode bytes from the beginning, leading to correct output:

  1d:   0f 1f 00                nopl   (%rax)
  20:   66 0f a3 d0             bt     %dx,%ax
  24:   e9 7f cd b5 e4          jmp    0xffffffffe4b5cda8
  29:*  0f 1f 80 00 00 00 00    nopl   0x0(%rax)                <-- trapping instruction
  30:   0f a3 d0                bt     %edx,%eax

While at it, make decodecode use bash as the interpreter - that thing
should be present on everything by now. It simplifies the code a lot
too.

Link: https://lkml.kernel.org/r/20220808085928.29840-1-bp@alien8.de
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>