Windows: make scheme_register_tls_space() always available

To make the API consistent for MSVC versus MinGW builds, make
a functional formerly required for embedding on 32-bit Windows
always available and required for all Windows variants.

pkgs/racket-doc/scribblings/inside/embedding.scrbl

@@ -87,7 +87,7 @@ To embed Racket CGC in a program, follow these steps:
   @cpp{scheme_main_stack_setup} trampoline registers the C stack with
   the memory manager without creating a namespace.)
 
-  On 32-bit Windows, when support for parallelism is enabled in the Racket
+  On Windows, when support for parallelism is enabled in the Racket
   build (as is the default), then before calling
   @cpp{scheme_main_setup}, your embedding application must first call
   @cppi{scheme_register_tls_space}:
@@ -101,7 +101,12 @@ To embed Racket CGC in a program, follow these steps:
 
   @verbatim[#:indent 2]{
    static __declspec(thread) void *tls_space;
-  }}
+  }
+
+  @history[#:changed "6.3" @elem{Calling @cpp{scheme_register_tls_space} is
+                                 required on all Windows variants, although the call
+                                 may be a no-op, depending on how Racket is
+                                 built.}]}
 
  @item{Configure the namespace by adding module declarations. The
   initial namespace contains declarations only for a few primitive

pkgs/racket-doc/scribblings/inside/memory.scrbl

@@ -899,7 +899,7 @@ overflow.}
            [void* ptr]
            [int   tls_index])]{
 
-Only available on 32-bit Windows; registers @var{ptr} as the address of a
+For Windows, registers @var{ptr} as the address of a
  thread-local pointer variable that is declared in the main
  executable. The variable's storage will be used to implement
  thread-local storage within the Racket run-time. See
@@ -908,7 +908,10 @@ Only available on 32-bit Windows; registers @var{ptr} as the address of a
 The @var{tls_index} argument must be @cpp{0}. It is currently
  ignored, but a future version may use the argument to allow
  declaration of the thread-local variable in a dynamically linked
- DLL.}
+ DLL.
+
+@history[#:changed "6.3" @elem{Changed from available only on 32-bit Windows
+                               to available on all Windows variants.}]}
 
 @function[(void scheme_register_static
            [void* ptr]

racket/src/racket/include/scheme.h

@@ -1982,9 +1982,7 @@ MZ_EXTERN int scheme_main_stack_setup(int no_auto_statics, Scheme_Nested_Main _m
 typedef int (*Scheme_Env_Main)(Scheme_Env *env, int argc, char **argv);
 MZ_EXTERN int scheme_main_setup(int no_auto_statics, Scheme_Env_Main _main, int argc, char **argv);
 
-#ifdef IMPLEMENT_THREAD_LOCAL_VIA_WIN_TLS
 MZ_EXTERN void scheme_register_tls_space(void *tls_space, int _tls_index);
-#endif
 
 MZ_EXTERN void scheme_register_static(void *ptr, intptr_t size);
 #if defined(MUST_REGISTER_GLOBALS) || defined(GC_MIGHT_USE_REGISTERED_STATICS)

racket/src/racket/src/salloc.c

@@ -223,6 +223,11 @@ Thread_Local_Variables *scheme_external_get_thread_local_variables() XFORM_SKIP_
 {
   return scheme_get_thread_local_variables();
 }
+#else
+void scheme_register_tls_space(void *tls_space, int tls_index) XFORM_SKIP_PROC
+{
+  /* Nothing to do; provided for compatibility. */
+}
 #endif
 
 #ifdef IMPLEMENT_THREAD_LOCAL_EXTERNALLY_VIA_PROC