rustbuild: allow building tools with debuginfo

rustbuild: allow building tools with debuginfo

Debugging information for the extended tools is currently disabled for
concerns about the size.  This patch adds `--enable-debuginfo-tools` to
let one opt into having that debuginfo.

This is useful for debugging the tools in distro packages.  We always
strip debuginfo into separate packages anyway, so the extra size is not
a concern in regular use.