Log Message:
mk: Cross-eyed hacks to support cross-libtool.

For a long time, when cross-building, say from native=amd64 to
target=powerpc, it was necessary to:

1. cross-build a _powerpc_ package called cross-libtool-base-powerpc,
   and then

2. install the powerpc package _natively_ with `pkg_add -m x86_64' to
   override the architecture check that normally forbids this kind of
   shenanigans,

in order to cross-build anything that uses libtool as a tool.

This is partly because libtool doesn't follow the normal GNU
convention of `./configure --build=<native platform> --host=<platform
package will run on> --target=<platform package is configured to
operate on>' -- in this example, build=amd64, host=amd64,
target=powerpc.

Instead, libtool expects to be cross-built itself, even if it's going
to run as a tool.  It's not as bonkers as it sounds at first: libtool
is just a shell script, and it caches various information about the
(cross-building!) toolchain it is built with so it can use that
information later when it is run as a tool itself to cross-compile
other software.

To make this work, we need to create the toolchain wrappers for
libtool _as if_ we were cross-building even if we are building a
native package.  So mk/tools uses a new flag TOOLS_USE_CROSS_COMPILE
instead of USE_CROSS_COMPILE, and libtool internally sets
MACHINE_ARCH=${TARGET_ARCH} (in the example above, powerpc) to make
it look like we're cross-building.  The new TOOLS_CROSS_DESTDIR is an
alias for the (defaulted) CROSS_DESTDIR, which must now be set
unconditionally in mk.conf in order for libtool to know where the
cross-destdir will be; _CROSS_DESTDIR remains empty when building any
native packages (including the native cross-libtool package).

Finally, we need to make the resulting package be a native package,
with MACHINE_ARCH set to the one that it will be installed on (in the
example above, amd64), so I added an indirection _BUILD_DEFS.${var}
to replace var on its own in the build definitions that get baked
into the package, shown by `pkg_info -B'.  Setting
_BUILD_DEFS.MACHINE_ARCH=${NATIVE_MACHINE_ARCH} ensures that this
mutant hybrid cross-built libtool still produces a native package.

All of this logic is gated on setting USE_CROSS_COMPILE in mk.conf or
LIBTOOL_CROSS_COMPILE in the package makefile, so it should be safe
for non-cross-builds -- when USE_CROSS_COMPILE=no and you're not
building cross-libtool, everything is as before.