Add a missing declaration to our unwind.h implementation. This is
necessary to be fully compatible with existing software that calls into
the linux unwind code. You can find documentation of this API and why it
exists in the discussion abot NPTL here:
https://gcc.gnu.org/ml/gcc-patches/2003-09/msg00154.html
Effectively revert r151058 which caused Clang's unwind.h to defer to
libunwind in all cases when installed.

At the time, Clang's unwind.h didn't provide huge chunks of the
LSB-specified unwind interface, and was generally too aenemic to use for
real software. However, it has since then become a strict superset of
the APIs provided by libunwind on Linux. Notably, you cannot compile
llgo's libgo library against libunwind, but you can against Clang's
unwind.h. So let's just use our header. =] I've checked pretty
thoroughly for any incompatibilities, and I am not aware of any.

An open question is whether or not we should continue to munge
GNU_SOURCE here. I didn't touch that as it potentially has compatibility
implications on systems I cannot easily test -- Darwin. If a Darwin
maintainer can verify that this is in fact unnecessary and remove it,
cool. Until then, leaving it in makes this change a no-op there, and
only really relevant on Linux systems where it is pretty clearly the
right way to go.