Guard init_priority attribute within libc++

Not all platforms support priority attribute. I'm moving conditional definition of this attribute to `include/__config`.

Reviewed By: #libc, aaron.ballman

Differential Revision: https://reviews.llvm.org/D91565

GitOrigin-RevId: 2c7e24c4b6893a93ddb2b2cca91eaf5bf7956965
diff --git a/include/__config b/include/__config
index 413931a..090d04c 100644
--- a/include/__config
+++ b/include/__config
@@ -1421,6 +1421,12 @@
 #define _LIBCPP_HAS_NO_FGETPOS_FSETPOS
 #endif
 
+#if __has_attribute(init_priority)
+# define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
+#else
+# define _LIBCPP_INIT_PRIORITY_MAX
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
diff --git a/src/experimental/memory_resource.cpp b/src/experimental/memory_resource.cpp
index 68c5bc9..1304ef3 100644
--- a/src/experimental/memory_resource.cpp
+++ b/src/experimental/memory_resource.cpp
@@ -76,16 +76,6 @@
   ~ResourceInitHelper() {}
 };
 
-// Detect if the init_priority attribute is supported.
-#if (defined(_LIBCPP_COMPILER_GCC) && defined(__APPLE__)) \
-  || defined(_LIBCPP_COMPILER_MSVC)
-// GCC on Apple doesn't support the init priority attribute,
-// and MSVC doesn't support any GCC attributes.
-# define _LIBCPP_INIT_PRIORITY_MAX
-#else
-# define _LIBCPP_INIT_PRIORITY_MAX __attribute__((init_priority(101)))
-#endif
-
 // When compiled in C++14 this initialization should be a constant expression.
 // Only in C++11 is "init_priority" needed to ensure initialization order.
 #if _LIBCPP_STD_VER > 11
diff --git a/src/iostream.cpp b/src/iostream.cpp
index d088593..ea95534 100644
--- a/src/iostream.cpp
+++ b/src/iostream.cpp
@@ -77,7 +77,7 @@
 #endif
 ;
 
-_LIBCPP_HIDDEN ios_base::Init __start_std_streams __attribute__((init_priority(101)));
+_LIBCPP_HIDDEN ios_base::Init __start_std_streams _LIBCPP_INIT_PRIORITY_MAX;
 
 // On Windows the TLS storage for locales needs to be initialized before we create
 // the standard streams, otherwise it may not be alive during program termination