gdb: LoongArch: prepend tramp frame unwinder for signal

Implement the "init" method of struct tramp_frame to prepend tramp
frame unwinder for signal on LoongArch.

With this patch, the following failed testcases can be fixed:

  FAIL: gdb.base/annota1.exp: backtrace @ signal handler (timeout)
  FAIL: gdb.base/annota3.exp: backtrace @ signal handler (pattern 2)

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>