LLVM does not currently emit these, but it is being discussed as an
option on platforms where `f32` is not hardware supported. Glibc/libgcc
also has the comparison functions [1] already.
The generic implementations for addition, subtraction, and
multiplication work for f16 without any complications, as do
comparisons, so add them here.
[1]: https://sourceware.org/git/?p=glibc.git;a=commit;h=6ec6c77867af4ddfec7323e0ac6ede89effca852
Emit `x86_no_sse` in the compiler-builtins (and builtins-test) build
script, and use it to simplify `all(target_arch = "x86",
not(target_fefature = "sse))` configuration.
