build: add glibc/libstdc++ back-compat stubs

glibc/libstdc++ have added new symbols in later releases. When running a new
binary against an older glibc, the run-time linker is unable to resolve the
new symbols and the binary refuses to run.

This can be fixed by adding our own versions of those functions, so that the
build-time linker does not emit undefined symbols for them.

This enables our binary releases to work on older Linux distros, while not
incurring the downsides of a fully static binary.
build: add an option for enabling glibc back-compat

Using "./configure --enable-glibc-back-compat" will attempt to be
compatible with a target running glibc abi 2.9 and libstdc++ abi 3.4.
gitian-linux: --enable-glibc-back-compat