[VCRUNTIME] Move compiler runtime headers into their own folder

These headers come with the MS compiler. Some of them are standard, like emmintrin.h, others are MS specific like crtdefs.h
This separation will allow using MS CRT headers. Eventually it can allow compiling with the compilers' runtime headers.

sdk/include/vcruntime/gcc_sal.h

@@ -0,0 +1,26 @@
+/*
+ * PROJECT:     ReactOS PSDK
+ * LICENSE:     MIT (https://spdx.org/licenses/MIT)
+ * PURPOSE:     Standard Annotation Language (SAL) definitions - GCC overrides
+ * COPYRIGHT:   2021 - Jérôme Gardou
+ */
+
+#pragma once
+
+#ifndef __GNUC__
+#error "Not for your compiler!"
+#endif
+
+#ifndef __has_attribute
+#pragma GCC warning "GCC without __has_attribute, no SAL niceties for you"
+#define __has_attribute(__x) 0
+#endif
+
+#if __has_attribute(warn_unused_result)
+# undef _Must_inspect_result_
+/* FIXME: Not really equivalent */
+# define _Must_inspect_result_ __attribute__((__warn_unused_result__))
+# undef _Check_return_
+/* This one is 1:1 equivalent */
+# define _Check_return_ __attribute__((__warn_unused_result__))
+#endif