Flip configure --enable-arch-native default (#2261)

Flip configure --enable-arch-native default (#2261)

Flip the default of configure's `--enable-arch-native` option from
enabled to disabled.

GCC's and clang's `-march=native` argument causes issues in some
environments, most notably containers where the compilers' heuristics
about CPU feature set may fail. These issues manifest as
hard-to-reproduce SIGILL errors in binaries such as `squid` or unit test
programs. The new default is safer. Performance-minded administrators
still have a convenient option to optimize via `--enable-arch-native`.