Remove __multc3

This C file doesn't seem to be compiled correctly, as it creates a reference to the x86 specific `__builtin_copysignq` on ARM. This intrinsic is unused and unavailable on Windows because it's for complex numbers Rust doesn't support, so it should be fine to remove.

The inclusion of the file seems to be the cause of some downstream link errors: rust-lang/rust#125619

build.rs

@@ -533,10 +533,6 @@ mod c {
                 ("__fe_getround", "fp_mode.c"),
                 ("__fe_raise_inexact", "fp_mode.c"),
             ]);
-
-            if target_os != "windows" {
-                sources.extend(&[("__multc3", "multc3.c")]);
-            }
         }
 
         if target_arch == "mips" || target_arch == "riscv32" || target_arch == "riscv64" {