double->single float conversion isn't a no-op on x87 anymore

 The conversion can result in overflow, so pass through a stack
 temporary to force a truncation.

 Test case by Peter Keller on sbcl-devel, 2013-06-26.
Store FP values from x87 to the heap outside pseudo-atomic

 It suffices to get the header right, and that way we avoid signaling
 FPEs in PA, when, as is bound to happen, a value is only truncated
 when it's boxed.
s/32/n-word-bits/ in bignum-index

 For some reason we missed that during all the work on x86-64. Then
 again, our static inference is good enough that no runtime check
 seem to be left to detect that the declaration is wrong.