Declare _availability_version_check as weak_import instead of looking it

up at runtime using dlsym

Calling dlsym with RTLD_DEFAULT can be very slow as all images in the
process are searched for the symbol.

Differential Revision: https://reviews.llvm.org/D150397

compiler-rt/lib/builtins/os_version_check.c

@@ -86,6 +86,10 @@ typedef Boolean (*CFStringGetCStringFuncTy)(CFStringRef, char *, CFIndex,
                                             CFStringEncoding);
 typedef void (*CFReleaseFuncTy)(CFTypeRef);
 
+extern __attribute__((weak_import))
+bool _availability_version_check(uint32_t count,
+                                 dyld_build_version_t versions[]);
+
 static void _initializeAvailabilityCheck(bool LoadPlist) {
   if (AvailabilityVersionCheck && !LoadPlist) {
     // New API is supported and we're not being asked to load the plist,
@@ -94,8 +98,8 @@ static void _initializeAvailabilityCheck(bool LoadPlist) {
   }
 
   // Use the new API if it's is available.
-  AvailabilityVersionCheck = (AvailabilityVersionCheckFuncTy)dlsym(
-      RTLD_DEFAULT, "_availability_version_check");
+  if (_availability_version_check)
+    AvailabilityVersionCheck = &_availability_version_check;
 
   if (AvailabilityVersionCheck && !LoadPlist) {
     // New API is supported and we're not being asked to load the plist,