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/mod.rs

@@ -300,6 +300,9 @@ pub struct CompiledCodeBase<T: CompilePhase> {
     /// This info is generated only if the `machine_code_cfg_info`
     /// flag is set.
     pub bb_edges: Vec<(CodeOffset, CodeOffset)>,
+    /// Minimum alignment for the function, derived from the use of any
+    /// pc-relative loads.
+    pub alignment: u32,
 }
 
 impl CompiledCodeStencil {
@@ -314,6 +317,7 @@ impl CompiledCodeStencil {
             dynamic_stackslot_offsets: self.dynamic_stackslot_offsets,
             bb_starts: self.bb_starts,
             bb_edges: self.bb_edges,
+            alignment: self.alignment,
         }
     }
 }
@@ -355,7 +359,7 @@ pub trait TextSectionBuilder {
     ///
     /// This function returns the offset at which the data was placed in the
     /// text section.
-    fn append(&mut self, labeled: bool, data: &[u8], align: Option<u32>) -> u64;
+    fn append(&mut self, labeled: bool, data: &[u8], align: u32) -> u64;
 
     /// Attempts to resolve a relocation for this function.
     ///