codegen: Configure `--expand-fp-convert-bits 64` on targets without `__int128_t` (#61439)
This prevents LLVM from emitting Libcalls to `__floatuntidf`,
`__floattidf`, `__fixdfti`, `__fixunsdfti` which are not included in
compiler-rt / libgcc on these platforms.
Resolves https://github.com/JuliaLang/julia/issues/61436. It would be
good to make the JIT error for similar bugs in the future instead of
hanging.