Add V4DFmode support to C

[Aldy Hernandez]

>>>>> "Geoffrey" == Geoffrey Keating <gkeating@apple.com> writes:

 > On Altivec, you can have 'vector float'.  Sometimes it's really useful
 > to have 'vector double' too, no matter how slow it might go.

 > Bootstrapped & tested on powerpc-darwin.  The actual new functionality
 > is very lightly tested and is certainly incomplete, but you can at
 > least do 'x = a + b'.

Could you add a simple testcase?  Now that we have generic simd
support, it should (in theory) work on all platforms.

Aldy