Fix math libraries not being linked on some platforms

This is a continuation/fix of 018616e. In that commit, we made it add
the math functions to all platforms (except apple-targets and windows),
and use `weak` linking, so that it can be used if the system doesn't
have those functions.

Didn't notice `mod math` was behind another set of `cfg`, so removed it
as well here.

library/compiler-builtins/src/lib.rs

@@ -45,14 +45,6 @@ mod macros;
 pub mod float;
 pub mod int;
 
-#[cfg(any(
-    all(target_family = "wasm", target_os = "unknown"),
-    target_os = "uefi",
-    target_os = "none",
-    target_os = "xous",
-    all(target_vendor = "fortanix", target_env = "sgx"),
-    target_os = "windows"
-))]
 pub mod math;
 pub mod mem;