[builtins] Move more float128-related helpers to GENERIC_TF_SOURCES list

There are two different _generic_ lists of source files in the compiler-rt/lib/builtins/CMakeLists.txt. Now there is no simple way to not use the tf-variants of helpers at all.

Since there exists a separate `GENERIC_TF_SOURCES` list, it seems quite natural to move all float128-related helpers there. If it is not possible for some reason, it would be useful to have an explanation of that reason somewhere near the `GENERIC_TF_SOURCES` definition.

Differential Revision: https://reviews.llvm.org/D81282