Add Xilinx Zynq ARM/FPGA SoC support to FreeBSD/arm port.

Submitted by:	Thomas Skibo <ThomasSkibo (at) sbcglobal.net>
Reviewed by:	wkoszek, freebsd-arm@ (no objections raised)
Merge libc++ trunk r180598.  Contains several minor cleanups and bug
fixes, no major changes.

MFC after:	2 weeks
Add Xilinx Zynq ARM/FPGA SoC support to FreeBSD/arm port.

Submitted by:	Thomas Skibo <ThomasSkibo (at) sbcglobal.net>
Tested by:	wkoszek (ZedBoard)
Reviewed by:	wkoszek, freebsd-arm@ (no objections raised)