Remove mulsf3.c and muldf3.c from compiler-rt.

mulsf3 and muldf3 have Rust implementations since 8970485. Remove the
C implementations to avoid conflicts.

build.rs

@@ -4987,9 +4987,7 @@ mod c {
                 "floatundisf.c",
                 "int_util.c",
                 "muldc3.c",
-                "muldf3.c",
                 "mulsc3.c",
-                "mulsf3.c",
                 "mulvdi3.c",
                 "mulvsi3.c",
                 "mulxc3.c",