[compiler-rt] Use uname syscall in GetKernelAreaSize()

Commit 5f5fb56c ("[compiler-rt] Intercept the uname() function")
broke clang-cmake-thumbv7-full-sh build:

http://lab.llvm.org:8011/builders/clang-cmake-thumbv7-full-sh/builds/4296

This also affects i386.

The reason is that intercepted uname() is called by GetKernelAreaSize()
during ASAN initialization on 32-bit platforms, but the respective
interceptor is not initialized yet at this point, leading to null
pointer dereference.

Introduce internal_uname() wrapper around uname syscall, and use it in
GetKernelAreaSize() and in FixedCVE_2016_2143().

Author: Ilya Leoshkevich
Reviewed By: Evgenii Stepanov

Differential Revision: https://reviews.llvm.org/D76776