Add symbols/cfi for assembly functions.

This is needed for some binary analysis programs (BOLT, in this case) to
find all the code.  A little ugly since we're putting labels that are
named as if they are local only in the global symbol table, but whatever,
there is plenty ugly and weird stuff in these assembly functions...

src/cpu.rs

@@ -26,6 +26,7 @@ pub(crate) struct Features(());
 pub(crate) fn features() -> Features {
     // We don't do runtime feature detection on iOS. instead some features are
     // assumed to be present; see `arm::Feature`.
+    #[cfg(not(target_env = "sgx"))]
     #[cfg(not(target_os = "ios"))]
     {
         static INIT: spin::Once<()> = spin::Once::new();