Target: pass and use locals by pointer instead of by value

This struct is larger than 256 bytes and code that copies it
consistently shows up in profiles of the compiler.

src/codegen/spirv/Module.zig

@@ -107,7 +107,7 @@ gpa: Allocator,
 arena: std.heap.ArenaAllocator,
 
 /// Target info
-target: std.Target,
+target: *const std.Target,
 
 /// The target SPIR-V version
 version: spec.Version,
@@ -187,7 +187,7 @@ decl_deps: std.ArrayListUnmanaged(Decl.Index) = .empty,
 /// The list of entry points that should be exported from this module.
 entry_points: std.AutoArrayHashMapUnmanaged(IdRef, EntryPoint) = .empty,
 
-pub fn init(gpa: Allocator, target: std.Target) Module {
+pub fn init(gpa: Allocator, target: *const std.Target) Module {
     const version_minor: u8 = blk: {
         // Prefer higher versions
         if (target.cpu.has(.spirv, .v1_6)) break :blk 6;