src/share/replaygain_synthesis/ : Remove old fast float math hack.

Old code was for the Intel compiler, had no appreciable performance benefits
and doesn't actually work with current version of that compiler.

Patch-from: lvqcl <lvqcl.mail@gmail.com>
replaygain_synthesis.c : Replace an int64_t/int32_t operation with a shift.

Patch-from: lvqcl <lvqcl.mail@gmail.com>