Fix build gdbserver build errors on arm, mips, aarch64.

Fix build gdbserver build errors on arm, mips, aarch64.

Fix build errors introduced by
https://sourceware.org/ml/gdb-patches/2015-05/msg00281.html, which
didn't account for the change of the name of the struct process_info
field 'private' to 'priv' made in
https://sourceware.org/ml/gdb-patches/2015-02/msg00829.html.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c (aarch64_linux_new_fork): Change reference
	to process_info.private to process_info.priv.
	* linux-arm-low.c (arm_new_fork): Likewise.
	* linux-mips-low.c (mips_linux_new_fork): Likewise.