Sparc64 compiler bug, fix requries ABI change

[Martin Husemann <martin%duskware.de@localhost>]

There is a bug in the in-tree gcc vs. the sparc ABI when passing structures
with floating point members of special size to functions.

The full story is here:

	https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69706

The short story: gcc ICEs for me when trying to build Firefox 53.

There are two variants of the bug: one with an ICE, the other with
silently generating non-ABI compliant code.

This is fixed in gcc 6, but never got backported by upstream due to the
ABI change.

I would prefer to ignore the ABI change (given the high chance we would have
run into the ICE variant instead of the silent one) and just backport
the change for our in-tree compiler.

Comments,

Martin