Define LIB2ADDEH_XTENSA_UNWIND_DW2_FDE to unwind-dw2-fde.c in
xtensa/t-elf and to unwind-dw2-fde-dip.c in xtensa/t-linux and use
LIB2ADDEH_XTENSA_UNWIND_DW2_FDE in LIB2ADDEH definition.

This fixes build for elf target with windowed xtensa core that currently
breaks with the following error message:
    
  unwind-dw2-fde-dip.c:36:40: fatal error: elf.h: No such file or directory

2016-10-18  Max Filippov  <jcmvbkbc@gmail.com>
libgcc/
	* config/xtensa/t-elf (LIB2ADDEH_XTENSA_UNWIND_DW2_FDE): New
	definition.
	* config/xtensa/t-linux (LIB2ADDEH_XTENSA_UNWIND_DW2_FDE): New
	definition.
	* config/xtensa/t-windowed (LIB2ADDEH): Use
	LIB2ADDEH_XTENSA_UNWIND_DW2_FDE defined by either xtensa/t-elf
	or xtensa/t-linux.

From-SVN: r241313