Install gcc's unwind.h instead of libexecinfo's

The prototypes in libexecinfo's unwind.h do not match those commonly
used (e.g. by gcc, clang, GNU libunwind, LLVM libunwind...), causing
C++ programs to fail to build on type mismatches (e.g. compiler-rt,
libc++abi).  Rather than providing our own header, reuse the one
included in gcc.

(mgorny)