[builtins] Avoid using CRT_LDBL_128BIT in implementation. NFC

[builtins] Avoid using CRT_LDBL_128BIT in implementation. NFC

Currently the *tf builtin functions can only be built if long double is an
IEEE float, which prevents them from being available e.g. for x86 targets
(unlike libgcc which has them). This non-functional change prepares the
builtins library *tf functions for being able to target x86 by decoupling
their presence from CRT_LDBL_128BIT and instead checking for a
CRT_HAS_TF_MODE macro. This change is NFC since the CRT_HAS_TF_MODE is
currently only set if long double is an IEEE 128-bit float.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D153812