Add libunwind support for Linux
Use backtrace_symbols_fd instead of backtrace_symbols

Conflicts:
	vm_dump.c

Conflicts:
	vm_dump.c
Dump x64 machine registers on sigsegv or sigbus

Conflicts:
	vm_dump.c

Conflicts:
	vm_dump.c
Clean up libunwind ifdef
Use backtrace_symbols_fd on signal

Conflicts:
	vm_dump.c
Merge branch 'libunwind_linux_shopify' into 2.1