Access to /proc/self/exe in linux-user mode

This is based on a recent bug in GCC Bugzilla: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60681

It looks like libbacktrace (GCC runtime library used for obtaining stack traces) uses /proc/self/exe for error reporting. Currently this is mapped to qemu-arm which effectively disables libbacktrace on linux-user.

It seems that QEMU already supports /proc/self/{maps,stat,auxv} so addition of /proc/self/exe may be trivial.