Add initial f16/f128 support

[beetrees]

This PR adds basic initial support for f16 and f128. While this allows simple programs (e.g. dbg!(1.0_f16)) to compile on architectures where Cranelift has basic support for f16/f128 values (currently x86-64 and AArch64), rustc_codegen_cranelift still can't compile compiler_builtins without no-f16-f128 due to missing support in cranelift_frontend (fixed by bytecodealliance/wasmtime#10631) and compiler_builtins requiring math support.

cc @tgross35

[tgross35]

LGTM for the non cranelift-specific parts, LLVM also does promote/demote for f16 powi and fmod.

[bjorn3]

Is Cranelift ABI compatible with LLVM for f16 and f128 across all targets Cranelift supports?

[beetrees]

By architecture/platform:

• Non-Windows x86-64 - f16/f128 are passed/returned in XMM registers as per specification.
• Windows x86-64 - f16/f128 are not supported by the Windows x86-64 ABI, but are available in Cranelift with enable_llvm_abi_extensions which currently matches LLVM 20 (passing/returning both types in XMM registers). The upcoming LLVM 21 changes the ABI to pass f128 indirectly (see [windows] Always pass fp128 arguments indirectly llvm/llvm-project#128848), so Cranelift will need to be updated to match.
• Non-Apple AArch64 - f16/f128 are passed/returned in floating-point registers as per specification.
• Apple AArch64 - the Apple ARM ABI is a variant of the standard AAPCS64, and Apple targets only support f16 in C (where it is passed/returned in floating-point registers). f128 is available in Cranelift with enable_llvm_abi_extensions, matching LLVM by passing/returning in floating-point registers like on non-Apple AArch64.
• 64-bit RISC-V - f16 is passed/returned in floating-point registers, f128 is passed/returned as a pair of integer registers, both as per the LP64D ABI in the specification (Cranelift does not currently support any other standard RISC-V ABIs).
• s390x (f16/f128 support not yet implemented in Cranelift) - f128 is passed/returned indirectly as per specification, f16 is only supported as an LLVM/Clang extension in the upcoming LLVM 21 (see [SystemZ] Add support for half (fp16) llvm/llvm-project#109164), where it is passed/returned in floating-point registers.

Summary: All currently implemented Cranelift f16/f128 support currently matches the standard ABIs, with enable_llvm_abi_extensions enabling extensions that match LLVM 20. Cranelift's Windows x86-64 f128 ABI (only available via enable_llvm_abi_extensions) will need updating to pass f128 indirectly to be compatible with LLVM 21.