Skip the float.pure.lisp/RANGE-REDUCTION test outside x86.

Skip the float.pure.lisp/RANGE-REDUCTION test outside x86.

The test was and is meant only to be used on x86 as it tests for results
the x87 FPU computes, which can differ from those on other platforms.
It was accidentally enabled for all platforms with commit
4c81c652cdc32faefee1bccb84c3c9a7854e3edd.

Revert this by skipping the test on everything except x86. Augment
the test's name and improve its comment so that it hopefully won't be
enabled again everywhere so easily.