CxxUtils: Don't try to use the x87 fsincos instruction any more.
Since a long time, we've had CxxUtils::sincos, which calculates sin and cos using the x87 fsincos instruction. At the time, this was significantly better than the alternatives. However, with current compilers on x86_64, it is in fact faster to use the sincos() library function than the x87 instruction. So remove the inlining bit. sincos() is still a GNU extension, so don't remove this entirely from CxxUtils --- this allow us to keep the #ifdefs in one place.