[builtins] Avoid using long double in generic sources (#69754)

Use of long double can be error-prone since it could be one of 80-bit
extended precision float, IEEE 128-bit float, or IBM 128-bit float.
Instead use an explicit xf_float typedef for the remaining cases where
long double is being used in the implementation. This patch does not
touch
the PPC specializations which still use long double.