perf symbol: Add LoongArch case in get_plt_sizes()

We can see the following definitions in bfd/elfnn-loongarch.c:

  #define PLT_HEADER_INSNS 8
  #define PLT_HEADER_SIZE (PLT_HEADER_INSNS * 4)

  #define PLT_ENTRY_INSNS 4
  #define PLT_ENTRY_SIZE (PLT_ENTRY_INSNS * 4)

so plt header size is 32 and plt entry size is 16 on LoongArch,
let us add LoongArch case in get_plt_sizes().

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: loongarch@lists.linux.dev
Cc: loongson-kernel@lists.loongnix.cn
Cc: Ingo Molnar <mingo@redhat.com>
Link: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elfnn-loongarch.c
Link: https://lore.kernel.org/r/1684835873-15956-1-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Namhyung Kim <namhyung@kernel.org>