* x86: improve x87 qword to float conversion, using single-precision constants saves space and removes need in separate load on FPU stack. No precision loss occurs because 2**64 is representable exactly even in single precision.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26941 3ad0048d-3df7-0310-abae-a5850022a9f2