Align functions according to their ISA's requirements (#4826)

Add a function_alignment function to the TargetIsa trait, and use it to align functions when generating objects. Additionally, collect the maximum alignment required for pc-relative constants in functions and pass that value out. Use the max of these two values when padding functions for alignment.

This fixes a bug on x86_64 where rip-relative loads to sse registers could cause a segfault, as functions weren't always guaranteed to be aligned to 16-byte addresses.

Fixes #4812

cranelift/codegen/src/machinst/vcode.rs

@@ -221,6 +221,9 @@ pub struct EmitResult<I: VCodeInst> {
 
     /// Stack frame size.
     pub frame_size: u32,
+
+    /// The alignment requirement for pc-relative loads.
+    pub alignment: u32,
 }
 
 /// A builder for a VCode function body.
@@ -1058,7 +1061,10 @@ impl<I: VCodeInst> VCode<I> {
         }
 
         // Emit the constants used by the function.
+        let mut alignment = 1;
         for (constant, data) in self.constants.iter() {
+            alignment = data.alignment().max(alignment);
+
             let label = buffer.get_label_for_constant(constant);
             buffer.defer_constant(label, data.alignment(), data.as_slice(), u32::max_value());
         }
@@ -1101,6 +1107,7 @@ impl<I: VCodeInst> VCode<I> {
             dynamic_stackslot_offsets: self.abi.dynamic_stackslot_offsets().clone(),
             value_labels_ranges,
             frame_size,
+            alignment,
         }
     }