[rtsan] Add basic test for c compilation (#107717)

I was reminded of
realtime-sanitizer/rtsan#25, where we were
including C++ code that was never linked in C.

Adding this test to ensure that we don't break this functionality in the
future

Author: cjappl

compiler-rt/test/rtsan/basic.cpp

@@ -1,4 +1,5 @@
 // RUN: %clangxx -fsanitize=realtime %s -o %t
+// RUN: %clang -fsanitize=realtime %s -o %t
 // RUN: not %run %t 2>&1 | FileCheck %s
 // UNSUPPORTED: ios