Serious bug in 4.9.0 (FP ctxt corruption on some ppc-e500 BSPs except mvme3100)

[Till Straumann]

The new multilib variant m8540 by default enables -mfloat-gprs=single
and is as such useless for now since we don't implement context-switching
the upper 32-bit of e500 GPRs.

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-> Any e500 MUST use -msoft-float
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(and may therefore as well use -mpowerpc -msoft-float)

There are several new BSPs which use -m8540 without soft-float which
will experience corruption of floating-point data since FP registers
are not properly saved/restored.

In the future -- once we have proper context-switching in place -- the
new multilib variant may indeed be useful but ATM it must not be used!


-- Till