csky: bugfix gdb coredump error.

In gdb/bfd elf32-csky.c, csky_elf_grok_prstatus() use fixed size of
elf_prstatus. It's 148 for abiv1 and 220 for abiv2, the size is enough
for coredump and no need full sizeof(struct pt_regs).

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reported-by: Lu Baoquan <lu.baoquan@intellif.com>
Reported-by: Liu Mao <liu.mao@intellif.com>