1. move sgidefs.h to libc/arch-mips.
See bionic 6e50cb454451916ced315680f6c13dc08cf3959a
2. change type of u_ar0 from "unsigned long" to "struct user_regs_struct*".
See bionic e03950fa0c5567edf70d011b856a027e03b1c0f7
3. sync fts.h, dlext.h, sys/user.h
4. remove ' from string.h
Change-Id: I2ba58b0df4c655f107ea086efbd0220de2b40e9a
The common header include/linux/a.out.h include asm/a.out.h, but
arm64/mips/mips64 don't have it. Add a placeholder before bionic
has it
Change-Id: I7084e0beabe86fec32987a89e66d6c094f1e8c22
