misc header changes for r10d

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
Merge "misc header changes for r10d"