headers: Redirect _environ to the _get_environ function in msvcrt.dll for arm/arm64

The direct _environ pointer is not available in msvcrt.dll on
arm/arm64. This provides a fallback read-only version of _environ,
but code that tries to write it still fails to compile though.

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-headers/crt/stdlib.h

diff --git a/mingw-w64-headers/crt/stdlib.h b/mingw-w64-headers/crt/stdlib.h
index 3fd81a5..45f0380 100644
--- a/mingw-w64-headers/crt/stdlib.h
+++ b/mingw-w64-headers/crt/stdlib.h
@@ -292,6 +292,31 @@
 #endif
 
 #ifndef _POSIX_
+#if (defined(_ARM_) || defined(__arm__) || defined(_ARM64_) || defined(__aarch64__)) && (__MSVCRT_VERSION__ == 0x700)
+  /* The plain msvcrt.dll for arm/aarch64 lacks _environ/_wenviron */
+  _CRTIMP void __cdecl _get_environ(char ***);
+  _CRTIMP void __cdecl _get_wenviron(wchar_t ***);
+
+  static __inline char **__get_environ_ptr(void) {
+    char **__ptr;
+    _get_environ(&__ptr);
+    return __ptr;
+  }
+
+  static __inline wchar_t **__get_wenviron_ptr(void) {
+    wchar_t **__ptr;
+    _get_wenviron(&__ptr);
+    return __ptr;
+  }
+
+#ifndef _environ
+#define _environ (__get_environ_ptr())
+#endif
+
+#ifndef _wenviron
+#define _wenviron (__get_wenviron_ptr())
+#endif
+#else /* ARM/ARM64 && __MSVCRT_VERSION__ == 0x700 */
 #ifndef _environ
   extern char *** __MINGW_IMP_SYMBOL(_environ);
 #define _environ (* __MINGW_IMP_SYMBOL(_environ))
@@ -301,6 +326,7 @@
   extern wchar_t *** __MINGW_IMP_SYMBOL(_wenviron);
 #define _wenviron (* __MINGW_IMP_SYMBOL(_wenviron))
 #endif
+#endif /* !ARM/ARM64 || __MSVCRT_VERSION__ != 0x700 */
 #endif /* !_POSIX_ */
 
 #ifndef _pgmptr