[build] Move unwinder to toplevel libgcc (v2)

[Joseph S. Myers]

This appears to have brought back PR 26478, build failure with readonly 
source directory:

cp unwind.h ../../.././gcc/include/unwind.h
cp: cannot create regular file `../../.././gcc/include/unwind.h': Permission denied
make[4]: *** [install-unwind_h] Error 1

-- 
Joseph S. Myers
joseph@codesourcery.com