[builtins] Support building the 128-bit float functions on ld80 platforms (#68132)

GCC provides these functions (e.g. __addtf3, etc.) in libgcc on x86_64.
Since Clang supports float128, we can also enable the existing code by
using float128 for fp_t if either __FLOAT128__ or __SIZEOF_FLOAT128__ is
defined instead of only supporting these builtins for platforms with
128-bit IEEE long doubles.
This commit defines a new tf_float typedef that matches a float with
attribute((mode(TF)) on each given architecture.

There are more tests that could be enabled for x86, but to keep the diff
smaller, I restricted test changes to ones that started failing as part
of this refactoring.

This change has been tested on x86 (natively) and
aarch64,powerpc64,riscv64 and sparc64 via qemu-user.

This supersedes https://reviews.llvm.org/D98261 and should also cover
the changes from https://github.com/llvm/llvm-project/pull/68041.

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