Fix symlink install hook

During the install, symlinks are added from libunwind-<arch> to
libunwind-generic.  However, on platforms that don't support
symlinking (such as Windows), the $(LN_S) macro is defined
as 'cp -p' instead.  This works fine, except that since the
target of the symlink is a relative path, the copy will only
succeed if the current directory is the directory that contains
the file.

The solution to this problem suggested in the Autotools manual
(see http://www.gnu.org/software/automake/manual/automake.html#Extending)
is to simply cd into the correct directory first.  This patch
makes that change for the symlinks that are being made during install.

[ edit: use relative path for the link name as well ]
Add basic support for the QNX operating system

This change adds some special cases to allow libunwind to compile
for QNX.

* QNX's copy of <elf.h> and <link.h> reside in sys/ instead.  To deal
  with this, an AC_CHECK_HEADERS() was added for both elf.h and link.h,
  with a special case to deal with QNX when they are not found in
  their default location.
* Similarly, QNX does not have <endian.h>.  In cases where the file is
  not found, logic was added to refer to QNX-specific macros to determine
  endianness.
* The QCC compiler, which is a wrapper around GCC, cannot handle some
  standard GCC options.  Therefore, logic was added to check for QCC,
  and when it is found, to suppress the use of -lgcc, and to express the
  option -nostartfiles as -Wc,-nostartfiles instead, which is correctly
  passed on to the underlying GCC.
* Finally, the support file os-qnx.c was added, patterned after the existing
  os-*.c files.  Only local image lookup is currently supported (see the
  comments for more information), but this is sufficient for QNX, since
  ptrace is not supported there anyway, and that is the only case where the
  function is required to do remote image lookup.

Change-Id: Ie7934f94a7317bdde59335f2acd4c3a97c0384c1