[SYCL][NFC] Mark external libc functions. (#19368)

<sycl/detail/defines_elementary.hpp> defines a __DPCPP_SYCL_EXTERNAL
macro for functions that are defined elsewhere. This macro is used both
for functions defined by the platform's system libraries and for
functions defined in libclc/libdevice. For Native CPU, these should be
treated differently: functions defined by the platform's system
libraries should be processed according to the platform's ABI, functions
defined by libclc/libdevice should be processed in a
host-target-independent way. Therefore, this PR introduces a
__DPCPP_SYCL_EXTERNAL_LIBC macro so that libc functions can be tagged as
such.

This PR does not yet make the changes to Native CPU to define
__DPCPP_SYCL_EXTERNAL and __DPCPP_SYCL_EXTERNAL_LIBC differently.

Author: hvdijk

sycl/include/sycl/builtins.hpp

@@ -15,13 +15,13 @@
 #ifdef __SYCL_DEVICE_ONLY__
 extern "C" {
 
-extern __DPCPP_SYCL_EXTERNAL void *memcpy(void *dest, const void *src,
-                                          size_t n);
-extern __DPCPP_SYCL_EXTERNAL void *memset(void *dest, int c, size_t n);
-extern __DPCPP_SYCL_EXTERNAL int memcmp(const void *s1, const void *s2,
-                                        size_t n);
-extern __DPCPP_SYCL_EXTERNAL int rand();
-extern __DPCPP_SYCL_EXTERNAL void srand(unsigned int seed);
+extern __DPCPP_SYCL_EXTERNAL_LIBC void *memcpy(void *dest, const void *src,
+                                               size_t n);
+extern __DPCPP_SYCL_EXTERNAL_LIBC void *memset(void *dest, int c, size_t n);
+extern __DPCPP_SYCL_EXTERNAL_LIBC int memcmp(const void *s1, const void *s2,
+                                             size_t n);
+extern __DPCPP_SYCL_EXTERNAL_LIBC int rand();
+extern __DPCPP_SYCL_EXTERNAL_LIBC void srand(unsigned int seed);
 extern __DPCPP_SYCL_EXTERNAL long long int __imf_llmax(long long int x,
                                                        long long int y);
 extern __DPCPP_SYCL_EXTERNAL long long int __imf_llmin(long long int x,
@@ -634,15 +634,15 @@ extern __DPCPP_SYCL_EXTERNAL int __imf_vimin_s32_relu(int x, int y);
 }
 #ifdef __GLIBC__
 namespace std {
-extern __DPCPP_SYCL_EXTERNAL void
+extern __DPCPP_SYCL_EXTERNAL_LIBC void
 __glibcxx_assert_fail(const char *file, int line, const char *func,
                       const char *cond) noexcept;
 } // namespace std
 extern "C" {
-extern __DPCPP_SYCL_EXTERNAL void __assert_fail(const char *expr,
-                                                const char *file,
-                                                unsigned int line,
-                                                const char *func);
+extern __DPCPP_SYCL_EXTERNAL_LIBC void __assert_fail(const char *expr,
+                                                     const char *file,
+                                                     unsigned int line,
+                                                     const char *func);
 }
 #elif defined(_WIN32)
 extern "C" {
@@ -652,8 +652,8 @@ extern "C" {
 //       APIs used by STL, such as _Cosh, are undocumented, even though
 //       they are open-sourced. Recognizing them as builtins is not
 //       straightforward currently.
-extern __DPCPP_SYCL_EXTERNAL void _wassert(const wchar_t *wexpr,
-                                           const wchar_t *wfile, unsigned line);
+extern __DPCPP_SYCL_EXTERNAL_LIBC void
+_wassert(const wchar_t *wexpr, const wchar_t *wfile, unsigned line);
 }
 #endif
 #endif // __SYCL_DEVICE_ONLY__

sycl/include/sycl/detail/defines_elementary.hpp

@@ -35,6 +35,10 @@
 #endif
 #endif
 
+#ifndef __DPCPP_SYCL_EXTERNAL_LIBC
+#define __DPCPP_SYCL_EXTERNAL_LIBC __DPCPP_SYCL_EXTERNAL
+#endif
+
 // Helper for enabling empty-base optimizations on MSVC.
 // TODO: Remove this when MSVC has this optimization enabled by default.
 #ifdef _MSC_VER