Move the cast_float_to_int fallback code to GCC

Now that we require at least LLVM 13, that codegen backend is always
using its intrinsic `fptosi.sat` and `fptoui.sat` conversions, so it
doesn't need the manual implementation. However, the GCC backend still
needs it, so we can move all of that code down there.

compiler/rustc_codegen_gcc/src/lib.rs

@@ -19,6 +19,7 @@
 #![warn(rust_2018_idioms)]
 #![warn(unused_lifetimes)]
 
+extern crate rustc_apfloat;
 extern crate rustc_ast;
 extern crate rustc_codegen_ssa;
 extern crate rustc_data_structures;