[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