Fixed single-threaded mode for Windows.

Added an option "single-threaded" to test the behaviour, and used
those commands for the testing:

 cd test/standalone/c_compiler
 zig build test
 zig build test -Dtarget=arm-linux-musleabi -Denable-qemu
 zig build test -Dtarget=wasm32-wasi -Denable-wasmtime
 zig build test -Dtarget=x86_64-windows-gnu -Denable-wine
 zig build test -Dtarget=x86_64-windows-gnu -Denable-wine -Dsingle-threaded
 zig build test -Dtarget=x86_64-macos-gnu -Denable-darling
 zig build test -Dtarget=x86_64-macos-gnu -Denable-darling -Dsingle-threaded

src/libcxx.zig

@@ -126,6 +126,12 @@ pub fn buildLibCXX(comp: *Compilation) !void {
             continue;
         if (std.mem.startsWith(u8, cxx_src, "src/support/ibm/") and target.os.tag != .zos)
             continue;
+        if (comp.bin_file.options.single_threaded) {
+            if (std.mem.startsWith(u8, cxx_src, "src/support/win32/thread_win32.cpp")) {
+                continue;
+            }
+            try cflags.append("-D_LIBCPP_HAS_NO_THREADS");
+        }
 
         try cflags.append("-DNDEBUG");
         try cflags.append("-D_LIBCPP_BUILDING_LIBRARY");
@@ -147,11 +153,6 @@ pub fn buildLibCXX(comp: *Compilation) !void {
             try cflags.append("-fno-exceptions");
         }
 
-        // WASM targets are single threaded.
-        if (comp.bin_file.options.single_threaded) {
-            try cflags.append("-D_LIBCPP_HAS_NO_THREADS");
-        }
-
         if (target.os.tag == .zos) {
             try cflags.append("-fno-aligned-allocation");
         } else {

test/standalone/c_compiler/build.zig

@@ -18,6 +18,7 @@ pub fn build(b: *Builder) void {
     const is_qemu_enabled = b.option(bool, "enable-qemu", "Use QEMU to run cross compiled foreign architecture tests") orelse false;
     const is_wasmtime_enabled = b.option(bool, "enable-wasmtime", "Use Wasmtime to enable and run WASI libstd tests") orelse false;
     const is_darling_enabled = b.option(bool, "enable-darling", "[Experimental] Use Darling to run cross compiled macOS tests") orelse false;
+    const single_threaded = b.option(bool, "single-threaded", "Test single threaded mode") orelse false;
 
     const test_step = b.step("test", "Test the program");
 
@@ -38,7 +39,7 @@ pub fn build(b: *Builder) void {
     exe_cpp.setBuildMode(mode);
     exe_cpp.setTarget(target);
     exe_cpp.linkLibCpp();
-    exe_cpp.single_threaded = isSingleThreadedTarget(target);
+    exe_cpp.single_threaded = isSingleThreadedTarget(target) or single_threaded;
     const os_tag = target.getOsTag();
     // macos C++ exceptions could be compiled, but not being catched,
     // additional support is required, possibly unwind + DWARF CFI