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_middle/src/middle/codegen_fn_attrs.rs

@@ -38,6 +38,9 @@ pub struct CodegenFnAttrs {
     /// be generated against a specific instruction set. Only usable on architectures which allow
     /// switching between multiple instruction sets.
     pub instruction_set: Option<InstructionSetAttr>,
+    /// The `#[repr(align(...))]` attribute. Indicates the value of which the function should be
+    /// aligned to.
+    pub alignment: Option<u32>,
 }
 
 bitflags! {
@@ -103,6 +106,7 @@ impl CodegenFnAttrs {
             link_section: None,
             no_sanitize: SanitizerSet::empty(),
             instruction_set: None,
+            alignment: None,
         }
     }