build: simplify dtrace configuration

The ability to pass libtool via $(CC) to dtrace and have it respect this
appears to be a feature that is only present in the systemtap version of
the tool.  In particular, FreeBSD (which seems to be using a copy of the
tool from Solaris) doesn't support this.

The result is that, with $(CC) ignored, and a .lo file specified in -o,
we get an ELF written to the .lo.

Instead of trying to have dtrace run libtool we can have libtool run
dtrace.  dtrace is really just a compiler that produces an object file
here, and it even understands -o, so libtool can make the appropriate
adjustments.

There appears to be some prior art for this approach.  A quick search
shows that at least QEMU is using this approach.  It also appears to
work on Linux with systemtap's dtrace and on FreeBSD.

This may regress cross-compilation because the dtrace command will have
no way of knowing which compiler we intend for it to use to produce the
object file.  I say "may" because I don't know if dtr...