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

Summary:
Commit 5f5fb56c ("[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