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/aarch64/mod.rs

@@ -95,6 +95,7 @@ impl TargetIsa for AArch64Backend {
             dynamic_stackslot_offsets,
             bb_starts: emit_result.bb_offsets,
             bb_edges: emit_result.bb_edges,
+            alignment: emit_result.alignment,
         })
     }
 
@@ -179,6 +180,12 @@ impl TargetIsa for AArch64Backend {
     fn map_regalloc_reg_to_dwarf(&self, reg: Reg) -> Result<u16, systemv::RegisterMappingError> {
         inst::unwind::systemv::map_reg(reg).map(|reg| reg.0)
     }
+
+    fn function_alignment(&self) -> u32 {
+        // We use 32-byte alignment for performance reasons, but for correctness we would only need
+        // 4-byte alignment.
+        32
+    }
 }
 
 impl fmt::Display for AArch64Backend {