[compiler-rt] Fall back to internal_uname() when called early

Summary:
Commit 5f5fb56c68 ("[compiler-rt] Intercept the uname() function")
broke sanitizer-x86_64-linux and clang-cmake-thumbv7-full-sh (again)
builds:

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/26313
http://lab.llvm.org:8011/builders/clang-cmake-thumbv7-full-sh/builds/4324

The reason is that uname() can be called as early as
__pthread_initialize_minimal_internal(). When intercepted, this
triggers ASan initialization, which eventually calls dlerror(), which
in turn uses pthreads, causing all sorts of issues.

Fix by falling back to internal_uname() when interceptor runs before
ASan is initialized. This is only for Linux at the moment.

Reviewers: eugenis, vitalybuka

Reviewed By: eugenis

Subscribers: dberris, #sanitizers, pcc

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76919

compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors.inc

@@ -9748,6 +9748,10 @@ INTERCEPTOR(int, sigaltstack, void *ss, void *oss) {
 
 #if SANITIZER_INTERCEPT_UNAME
 INTERCEPTOR(int, uname, struct utsname *utsname) {
+#if SANITIZER_LINUX
+  if (COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED)
+    return internal_uname(utsname);
+#endif
   void *ctx;
   COMMON_INTERCEPTOR_ENTER(ctx, uname, utsname);
   int res = REAL(uname)(utsname);