Revert "[compiler-rt] simplifying ::ReExec for freebsd. (llvm#79711)" (llvm#82933)

This reverts commit 691b12a.

Author: devnexen

compiler-rt/lib/sanitizer_common/sanitizer_getauxval.h

@@ -55,20 +55,6 @@ static inline decltype(AuxInfo::a_v) getauxval(decltype(AuxInfo::a_type) type) {
   return 0;
 }
 
-#elif SANITIZER_FREEBSD
-#  include <sys/auxv.h>
-
-#  define SANITIZER_USE_GETAUXVAL 1
-#  define AT_EXECFN AT_EXECPATH
-
-static inline unsigned long getauxval(unsigned long type) {
-  unsigned long buf = 0ul;
-
-  if (elf_aux_info(static_cast<int>(type), &buf, sizeof(buf)))
-    return 0ul;
-  return buf;
-}
-
 #endif
 
 #endif // SANITIZER_GETAUXVAL_H

compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp

@@ -934,7 +934,14 @@ u64 MonotonicNanoTime() {
 void ReExec() {
   const char *pathname = "/proc/self/exe";
 
-#  if SANITIZER_NETBSD
+#  if SANITIZER_FREEBSD
+  for (const auto *aux = __elf_aux_vector; aux->a_type != AT_NULL; aux++) {
+    if (aux->a_type == AT_EXECPATH) {
+      pathname = static_cast<const char *>(aux->a_un.a_ptr);
+      break;
+    }
+  }
+#  elif SANITIZER_NETBSD
   static const int name[] = {
       CTL_KERN,
       KERN_PROC_ARGS,