powerpc/64: Drop ppc_inst_as_str()

The ppc_inst_as_str() macro tries to make printing variable length,
aka "prefixed", instructions convenient. It mostly succeeds, but it does
hide an on-stack buffer, which triggers stack protector.

More problematically it doesn't compile at all with GCC 12,
with -Wdangling-pointer, due to the fact that it returns the char buffer
declared inside the macro:

  arch/powerpc/kernel/trace/ftrace.c: In function '__ftrace_modify_call':
  ./include/linux/printk.h:475:44: error: using a dangling pointer to '__str' [-Werror=dangling-pointer=]
    475 | #define printk(fmt, ...) printk_index_wrap(_printk, fmt, ##__VA_ARGS__)
    ...
  arch/powerpc/kernel/trace/ftrace.c:567:17: note: in expansion of macro 'pr_err'
    567 |                 pr_err("Not expected bl: opcode is %s\n", ppc_inst_as_str(op));
        |                 ^~~~~~
  ./arch/powerpc/include/asm/inst.h:156:14: note: '__str' declared here
    156 |         char __str[PPC_INST_STR_LEN];   \
        |              ^~~~~

This could be fixed by having the caller declare the buffer, but in some
places there'd need to be two buffers. In all cases where
ppc_inst_as_str() is used the output is not really meant for user
consumption, it's almost always indicative of a kernel bug.

A simpler solution is to just print the value as an unsigned long. For
normal instructions the output is identical. For prefixed instructions
the value is printed as a single 64-bit quantity, whereas previously the
low half was printed first. But that is good enough for debug output,
especially as prefixed instructions will be rare in kernel code in
practice.

Old:
  c000000000111170  60420000      ori     r2,r2,0
  c000000000111174  04100001 e580fb00     .long 0xe580fb0004100001

New:
  c00000000010f90c  60420000      ori     r2,r2,0
  c00000000010f910  e580fb0004100001      .long 0xe580fb0004100001

Reported-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reported-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220531065936.3674348-1-mpe@ellerman.id.au