Enable float80/float128 on Unsupported Targets for LLVM libc

This proposal is based on the LLVM project for GSoC 2026 with the same title, from Tue Ly, Nicolas Celik and Krishna Pandey. Discourse: https://discourse.llvm.org/t/gsoc-2026-libc-enable-float80-and-float128-math-support-on-unsupported-targets-for-llvm-libc/89647. Float80 (x87 80-bit extended precision) and Float128 (IEEE 754 binary128 quad precision) are high-precision floating-point formats critical for correctly rounded math implementations. However, not all compilers or architectures support these types natively which prevents LLVM libc from building and testing its high-precision math routines on these platforms. The goal of this project is to implement software support for these types in LLVM libc in a compiler and architecture agnostic way. Deliverables: - Implement float80 and float128 in a compiler and architecture agnostic way. Implement generic basic arithmetic operations that work on all supported architectures. - Demonstrate compiling and passing tests on unsupported targets (float80 on Mac ARM64, float128 on MSVC).