objtool/LoongArch: Mark special atomic instruction as INSN_BUG type

When compiling with LLVM and CONFIG_RUST is set, there exists the
following objtool warning:

  rust/compiler_builtins.o: warning: objtool: __rust__unordsf2(): unexpected end of section .text.unlikely.

objdump shows that the end of section .text.unlikely is an atomic
instruction:

  amswap.w        $zero, $ra, $zero

According to the LoongArch Reference Manual, if the amswap.w atomic
memory access instruction has the same register number as rd and rj,
the execution will trigger an Instruction Non-defined Exception, so
mark the above instruction as INSN_BUG type to fix the warning.

Cc: stable@vger.kernel.org
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>