This libbacktrace patch changes the loop over dynamic libraries on
Mach-O to keep going if we fail to find the debug info for a
particular library.  We can still pick up debug info for other
libraries even if one fails.  Tested on x86_64-pc-linux-gnu which
admittedly does little, but others have tested it on Mach-o.
Committed to mainline.

Ian

* macho.c (backtrace_initialize) [HAVE_MACH_O_DYLD_H]: Don't exit
loop if we can't find debug info for one shared library.