Synchronizes extern(C) bindings with latest release.

Reviewed-on: https://github.com/dlang/druntime/pull/2741

libphobos/ChangeLog:

	* libdruntime/Makefile.am (DRUNTIME_DSOURCES_BIONIC): Add
	core/sys/bionic/string.d.
	(DRUNTIME_DSOURCES_DARWIN): Add core/sys/darwin/string.d.
	(DRUNTIME_DSOURCES_DRAGONFLYBSD): Add core/sys/dragonflybsd/string.d,
	core/sys/dragonflybsd/sys/socket.d.
	(DRUNTIME_DSOURCES_FREEBSD): Add core/sys/freebsd/string.d.
	(DRUNTIME_DSOURCES_LINUX): Add core/sys/linux/string.d.
	(DRUNTIME_DSOURCES_NETBSD): Add core/sys/netbsd/string.d,
	core/sys/netbsd/sys/featuretest.d.
	* libdruntime/Makefile.in: Regenerate.

From-SVN: r274773