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

crates/environ/src/compilation.rs

@@ -28,6 +28,9 @@ pub struct FunctionInfo {
     pub start: u64,
     /// The size of the compiled function, in bytes.
     pub length: u32,
+
+    /// The alignment requirements of this function, in bytes.
+    pub alignment: u32,
 }
 
 /// Information about a compiled trampoline which the host can call to enter