[libunwind] Partially revert r297174 to fix build on at least FreeBSD.
The changes in r297174 moved the #include of <link.h> on FreeBSD (and
probably other systems) inside of the open 'libunwind' namespace
causing various system-provided types such as pid_t to be declared in
this namespace rather than the global namespace. Fix this by moving
the relevant declarations before the 'libunwind' namespace is opened,
but still using the cleaned up declarations from r297174.
Reviewed By: ed, compnerd
Differential Revision: https://reviews.llvm.org/D38108