Fix compare of reals.

[redstar]

Comparing 2 reals for equality is done with memcmp(). The length was hardcoded to 10
which is wrong on all platforms with 64bit reals.

[dnadlinger]

Port depending on Target strikes me as a dependency that is entirely the wrong way around. Port is supposed to be concerned with host functions, not target ones.

[redstar]

It is simply a sign that dmd has no real distinction between host and target. The floating point machinery is totally target dependent but the details are spread over the source: the longdouble class, the Target class and the Port functions all contribute to this machinery.