[sanitizer] Expose getauxval as weak declaration to fix TSanitizer-x86_64-Test-Nolibc

[MaskRay]

Summary

After rCRT346215, TSanitizer-x86_64-Test-Nolibc does not link as getauxval() is a libc dependency.

This patch unifies include <sys/auxv.h> and weak declaration to just use
the weak declaration. If SANITIZER_USE_GETAUXVAL is 1, getauxval() is
guaranteed to be non-null and we can use it for AT_BASE AT_EXECFN and
other purposes.

[bkramer]

Thanks! This looks good to me, but I'd like someone from sanitizer team review it too.

[vitalybuka]

I think we should move ReExec into libcdep file D54163

lib/sanitizer_common/sanitizer_getauxval.h

 
 # if __GLIBC_PREREQ(2, 16) || (SANITIZER_ANDROID && __ANDROID_API__ >= 21) || \
      SANITIZER_FUCHSIA
 #  define SANITIZER_USE_GETAUXVAL 1
 # else
 #  define SANITIZER_USE_GETAUXVAL 0
 # endif
 
-# if SANITIZER_USE_GETAUXVAL
-#  include <sys/auxv.h>
-# else
-// The weak getauxval definition allows to check for the function at runtime.
-// This is useful for Android, when compiled at a lower API level yet running
-// on a more recent platform that offers the function.
+// The weak getauxval definition allows to check for the function at runtime. It
+// guarantees to be non-null if SANITIZER_USE_GETAUXVAL is 1. This is useful for
+// Android, when compiled at a lower API level yet running on a more recent
+// platform that offers the function.
 extern "C" SANITIZER_WEAK_ATTRIBUTE
 unsigned long getauxval(unsigned long type);  // NOLINT
-# endif
 
 #endif // SANITIZER_LINUX || SANITIZER_FUCHSIA
 
 #endif // SANITIZER_GETAUXVAL_H