linux-user/mips64: set EF_MIPS_ABI2 in mipsn32 core dumps

mipsn32 core files had e_flags 0, so gdb took them for o32, found no
general-purpose registers and failed with an internal error in
frame_unwind_find_by_frame.

Linux sets e_flags to EF_MIPS_ABI2 for n32 cores (user_mipsn32_view in
arch/mips/kernel/ptrace.c).

Signed-off-by: Helge Deller <deller@gmx.de>