MIPS: 64-bit: Fix o32 core dump

If an o32 process generates a core dump on a 64 bit kernel, the core file
will not be correctly recognized. This is because ELF_CORE_COPY_REGS and
ELF_CORE_COPY_TASK_REGS are not correctly defined for o32 and will use
the default register set which would be CONFIG_64BIT in asm/elf.h.

So we'll switch to use the right register defines in this situation by
checking for WANT_COMPAT_REG_H and use the right defines of
ELF_CORE_COPY_REGS and ELF_CORE_COPY_TASK_REGS.

[Ralf: made ELF_CORE_COPY_TASK_REGS() bullet-proof against funny arguments.]

Signed-off-by: Yong Zhang <yong.zhang@windriver.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>