MSVC 19.25 complains about signbit being ambiguous between
signbit(float) and signbit(double) overloads when called with an int8_t.
To remove the ambiguity, cast to a double.

Change-Id: I698f05eed9248eef493bbe46b75fcd07e37e2a05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2118510Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#66856}