libgo: Use libbacktrace rather than debug/elf registration.

ChangeLog

Makefile.def

Makefile.in

libgo/Makefile.am

libgo/Makefile.in

libgo/go/debug/elf/elf_test.go

libgo/go/debug/elf/file_test.go

libgo/go/debug/elf/runtime.go

libgo/go/net/http/pprof/pprof.go

libgo/go/net/ip_test.go

libgo/go/runtime/debug/stack.go

libgo/go/runtime/pprof/pprof.go

libgo/go/testing/testing.go

libgo/runtime/go-caller.c

libgo/runtime/go-callers.c

libgo/runtime/runtime.h