Quadmath doesn't work on PPC

On a Power9 machine:

```
julia> using DoubleFloats
[ Info: Precompiling DoubleFloats [497a8b3b-efae-58df-a0af-a86822472b78]
ERROR: LoadError: ccall: could not find function __floatsitf in library libgcc_s.so.1
Stacktrace:
[1] Quadmath.Float128(::Int32) at /home/vchuravy/.julia/packages/Quadmath/6EO4T/src/Quadmath.jl:144
[2] top-level scope at /home/vchuravy/.julia/packages/Quadmath/6EO4T/src/Quadmath.jl:271
[3] include at ./boot.jl:328 [inlined]
[4] include_relative(::Module, ::String) at ./loading.jl:1105
[5] include(::Module, ::String) at ./Base.jl:31
[6] top-level scope at none:2
[7] eval at ./boot.jl:330 [inlined]
[8] eval(::Expr) at ./client.jl:425
[9] top-level scope at ./none:3
in expression starting at /home/vchuravy/.julia/packages/Quadmath/6EO4T/src/Quadmath.jl:265
```

I found https://gcc.gnu.org/wiki/Ieee128PowerPC

In general it might be more reliable to use LLVM `f128` support instead of doing target specific lowering in a package?