linux: Add /usr/lib/debug symbol path checking
linux: Add /usr/lib/debug symbol path checking

Debian likes to hive off symbol information to separate -dbg packages,
installing the debug information in a parallel tree rooted at /usr/lib/debug.
libunwind doesn't know how to find these files but it's easy, per the tested,
attached patch, to make it so. I don't see /usr/lib/debug at
http://www.pathname.com/fhs/pub/fhs-2.3.html#USRLIBLIBRARIESFORPROGRAMMINGANDPA
but
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Debugging_with_gdb/separate-debug-files.html
says it's not just Debian and hints it might be driven by gdb. I do see mention
of /usr/lib/debug in libunwind already, but only in a DWARF-specific part of
the code.  A minor concern might be checking errno to see if the problem is
ENOENT before trying the alternate path. That might be better than flogging a
mmap problem or being out of file handles or whatever.

libunwind/libunwind,djwatson/libunwind,yuyichao/libunwind,libunwind/libunwind,djwatson/libunwind,unkadoug/libunwind,frida/libunwind,rantala/libunwind,unkadoug/libunwind,frida/libunwind,frida/libunwind,libunwind/libunwind,vtjnash/libunwind,unkadoug/libunwind,vtjnash/libunwind,djwatson/libunwind,yuyichao/libunwind,vtjnash/libunwind,yuyichao/libunwind,rantala/libunwind,rantala/libunwind