Revert "[CMake] CheckAtomic.cmake: catch false positives in RISC-V"

This reverts commit a572a8a.

Apparently it was part of a larger series, and I'm not planning on
merging that; see https://reviews.llvm.org/D68964

Author: zmodem

llvm/cmake/modules/CheckAtomic.cmake

@@ -12,12 +12,8 @@ function(check_working_cxx_atomics varname)
   CHECK_CXX_SOURCE_COMPILES("
 #include <atomic>
 std::atomic<int> x;
-std::atomic<short> y;
-std::atomic<char> z;
 int main() {
-  ++z;
-  ++y;
-  return ++x;
+  return x;
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})