Auto merge of #81234 - repnop:fn-alignment, r=lcnr

Allow specifying alignment for functions

Fixes #75072

This allows the user to specify alignment for functions, which can be useful for low level work where functions need to necessarily be aligned to a specific value.

I believe the error cases not covered in the match are caught earlier based on my testing so I had them just return `None`.

compiler/rustc_span/src/symbol.rs

@@ -561,6 +561,7 @@ symbols! {
         fmt,
         fmt_internals,
         fmul_fast,
+        fn_align,
         fn_must_use,
         fn_mut,
         fn_once,