Add Program#alignment_byte_width

Don't hardcode the alignment to 4 or 8 when the actual value depends on
the actual target (e.g. it's always 1 for AVR targets).

src/compiler/crystal/codegen/primitives.cr

@@ -1257,7 +1257,7 @@ class Crystal::CodeGenVisitor
     when CharType
       inst.alignment = 4
     else
-      inst.alignment = @program.bits64? ? 8 : 4
+      inst.alignment = @program.alignment_byte_width
     end
   end
 

src/compiler/crystal/codegen/target.cr

@@ -75,6 +75,19 @@ class Crystal::Codegen::Target
     end
   end
 
+  def alignment_byte_width
+    case @architecture
+    when "aarch64", "x86_64"
+      8
+    when "arm", "i386", "wasm32"
+      4
+    when "avr"
+      1
+    else
+      raise "BUG: unknown Target#alignment_byte_width for #{@architecture} target architecture"
+    end
+  end
+
   def os_name
     case self
     when .macos?

src/compiler/crystal/semantic/flags.cr

@@ -29,6 +29,10 @@ class Crystal::Program
     codegen_target.size_bit_width
   end
 
+  def alignment_byte_width
+    codegen_target.alignment_byte_width
+  end
+
   private def flags_for_target(target)
     flags = Set(String).new