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/isa/x64/mod.rs

@@ -88,6 +88,7 @@ impl TargetIsa for X64Backend {
             dynamic_stackslot_offsets,
             bb_starts: emit_result.bb_offsets,
             bb_edges: emit_result.bb_edges,
+            alignment: emit_result.alignment,
         })
     }
 
@@ -158,6 +159,12 @@ impl TargetIsa for X64Backend {
     fn text_section_builder(&self, num_funcs: u32) -> Box<dyn TextSectionBuilder> {
         Box::new(MachTextSectionBuilder::<inst::Inst>::new(num_funcs))
     }
+
+    /// Align functions on x86 to 16 bytes, ensuring that rip-relative loads to SSE registers are
+    /// always from aligned memory.
+    fn function_alignment(&self) -> u32 {
+        16
+    }
 }
 
 impl fmt::Display for X64Backend {