Darwin, config: Revise host config fragment.

Darwin, config: Revise host config fragment.

There were two uses for the Darwin host config fragment:

The first is to arrange for targets that support mdynamic-no-pic
to be built with that enabled (since it makes a significant
difference to the compiler performance).  We can be more specific
in the application of this, since it only applies to 32b hosts
plus powerpc64-darwin9.

The second was to work around a tool bug where -fno-PIE was not
propagated to the link stage.  This second use is redundant,
since the buggy toolchain cannot bootstrap current GCC sources
anyway.

This makes the host fragment more specific and reduces the number
of toolchains for which it is included which reduces clutter in
configure lines.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
config/
	* mh-darwin: Make this specific to handling the
	mdynamic-no-pic case.