CLI: add --host-target, --host-cpu, and --host-dynamic-linker

These are advanced options that make it possible to simultaneously
target the "host" computer, while overriding assumptions about the host
computer such as what CPU features are available, or what OS version
range to target.

This is useful only for the case of integrating with system package
manager builds which want to pretend the host is also the target.

Author: andrewrk

src/main.zig

@@ -967,6 +967,9 @@ fn buildOutputType(
         .libc_paths_file = try EnvVar.ZIG_LIBC.get(arena),
         .link_objects = .{},
         .native_system_include_paths = &.{},
+        .host_triple = null,
+        .host_cpu = null,
+        .host_dynamic_linker = null,
     };
 
     // before arg parsing, check for the NO_COLOR environment variable
@@ -1258,6 +1261,12 @@ fn buildOutputType(
                         mod_opts.optimize_mode = parseOptimizeMode(arg["-O".len..]);
                     } else if (mem.eql(u8, arg, "--dynamic-linker")) {
                         create_module.dynamic_linker = args_iter.nextOrFatal();
+                    } else if (mem.eql(u8, arg, "--host-target")) {
+                        create_module.host_triple = args_iter.nextOrFatal();
+                    } else if (mem.eql(u8, arg, "--host-cpu")) {
+                        create_module.host_cpu = args_iter.nextOrFatal();
+                    } else if (mem.eql(u8, arg, "--host-dynamic-linker")) {
+                        create_module.host_dynamic_linker = args_iter.nextOrFatal();
                     } else if (mem.eql(u8, arg, "--sysroot")) {
                         const next_arg = args_iter.nextOrFatal();
                         create_module.sysroot = next_arg;
@@ -3447,6 +3456,9 @@ const CreateModule = struct {
     each_lib_rpath: ?bool,
     libc_paths_file: ?[]const u8,
     link_objects: std.ArrayListUnmanaged(Compilation.LinkObject),
+    host_triple: ?[]const u8,
+    host_cpu: ?[]const u8,
+    host_dynamic_linker: ?[]const u8,
 };
 
 fn createModule(
@@ -3531,7 +3543,15 @@ fn createModule(
         }
 
         const target_query = parseTargetQueryOrReportFatalError(arena, target_parse_options);
-        const target = resolveTargetQueryOrFatal(target_query);
+        const adjusted_target_query = a: {
+            if (!target_query.isNative()) break :a target_query;
+            if (create_module.host_triple) |triple| target_parse_options.arch_os_abi = triple;
+            if (create_module.host_cpu) |cpu| target_parse_options.cpu_features = cpu;
+            if (create_module.host_dynamic_linker) |dl| target_parse_options.dynamic_linker = dl;
+            break :a parseTargetQueryOrReportFatalError(arena, target_parse_options);
+        };
+
+        const target = resolveTargetQueryOrFatal(adjusted_target_query);
         break :t .{
             .result = target,
             .is_native_os = target_query.isNativeOs(),