Bug 1080077 - For sandbox failures with no crash reporter, log the C stack. r=kang

This is mostly for ASAN builds, which --disable-crash-reporter, but also
fixes a related papercut: debug builds don't use the crash reporter
unless overridden with an environment variable.

Note: this is Linux-only, so NS_StackWalk is always part of the build;
see also bug 1063455.