using `uname -s`-`uname -r`-`uname -p || uname -m`. This can be overridden
in mk.conf or the environment, and is used by host tools to ensure that
build hosts do not clobber each other.
* Now that there's an objdir in src/tools, make a default TOOLDIR by creating
"tools.${HOST_OSTYPE}" in the objdir (if it exists) of src/tools. This
means that TOOLDIR is no longer required to be set in a build, though it
still may be set manually if desired.
* Set MKTOOLS to "yes" by default, since the default TOOLDIR is now located
in a known mutable location. This may still be set manually if desired.
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