cmd/compile: skip float32 constant folding test on 387 builder

cmd/compile: skip float32 constant folding test on 387 builder

The 387 unit always quietens float64 and float32 signaling NaNs,
even when just loading and storing them. This makes it difficult
to propagate such values in the compiler. This is a hard problem
to fix and it is also very obscure.

Updates #27516.

Change-Id: I03d88e31f14c86fa682fcea4b6d1fba18968aee8
Reviewed-on: https://go-review.googlesource.com/135195
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>