Remove the need for #[cfg] in #[use_c_shim_if]

This commit tweaks the implementation of the synthetic
`#[use_c_shim_if]` attribute, renaming it to
`#[maybe_use_optimized_c_shim]` in the process. This no longer requires
specifying a `#[cfg]` clause indicating when the optimized intrinsic
should be used, but rather this is inferred and printed from the build
script.

The build script will now print out appropriate `#[cfg]` directives for
rustc to indicate what intrinsics it's compiling. This should remove the
need for us to keep the build script and the source in sync, but rather
the build script can simply take care of everything.

library/compiler-builtins/src/int/mod.rs

@@ -302,7 +302,7 @@ impl_wide_int!(u32, u64, 32);
 impl_wide_int!(u64, u128, 64);
 
 intrinsics! {
-    #[use_c_shim_if(/* always if C compilation is enabled */)]
+    #[maybe_use_optimized_c_shim]
     #[cfg(any(
         target_pointer_width = "16",
         target_pointer_width = "32",