Centralize gnu_debuglink logic in elfxx. Remove previous duplicated logic
from Gfind_proc_info and os-linux.
Logic is roughly the same as previous load_debug_frame, but uses VLAs
instead of malloc.
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.
Currently, libunwind allocates several PATH_MAX entries on stack, while
trying to find a binary via /proc/.../maps.
However stack space may be at premium (especially when sigaltstack is used),
and PATH_MAX on Linux is 4096, while SIGSTKSZ is only 8192 on x86.
Attached patch eliminates multiple PATH_MAX stack allocations, and simplifies
code in maps_next, at the cost of being unable to do anything if we can't
mmap one page. It appears to me that under such low-memory conditions,
libunwind will fail shortly elsewhere anyway.
This patch also disables more of debug_frame-handling code when
CONFIG_DEBUG_FRAME is undefined.
Tested on Linux/x86_64 with and without CONFIG_DEBUG_FRAME, no regressions.
