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/mod.rs

@@ -277,6 +277,9 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     /// will be "labeled" or might have calls between them, typically the number
     /// of defined functions in the object file.
     fn text_section_builder(&self, num_labeled_funcs: u32) -> Box<dyn TextSectionBuilder>;
+
+    /// The function alignment required by this ISA.
+    fn function_alignment(&self) -> u32;
 }
 
 /// Methods implemented for free for target ISA!