allow coercion of large fixnums to floats outside x86

allow coercion of large fixnums to floats outside x86

 The reason we need guard against this on x86 is due to the FPU there always
 using double-precision internally, which can lead to us deriving an
 inconsistent type unless the fixnum is exactly represented by a single-float.

 However, no such danger exists outside x86.

 (Test-suite already contains tests for this.)