Explicitly check for atomic addition and relaxed load operation support

...and properly find and link against `libatomic` using find_library.
This fixes the qtdeclarative build on the RISC-V platform.

Initial-patch-by: Sprite <SpriteOvO@gmail.com>
Pick-to: 6.2
Pick-to: 6.3
Task-number: QTBUG-99234
Change-Id: I2b5e4812886ce45cb02bed3106ce8c519b294cbe
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

cmake/FindWrapAtomic.cmake

@@ -10,35 +10,39 @@ include(CheckCXXSourceCompiles)
 set (atomic_test_sources "#include <atomic>
 #include <cstdint>
 
-void test(volatile std::atomic<std::int64_t> &a)
-{
-    std::int64_t v = a.load(std::memory_order_acquire);
-    while (!a.compare_exchange_strong(v, v + 1,
-                                      std::memory_order_acq_rel,
-                                      std::memory_order_acquire)) {
-        v = a.exchange(v - 1);
-    }
-    a.store(v + 1, std::memory_order_release);
-}
-
 int main(int, char **)
 {
-    void *ptr = (void*)0xffffffc0; // any random pointer
+    volatile std::atomic<char>    size_1;
-    test(*reinterpret_cast<std::atomic<std::int64_t> *>(ptr));
+    volatile std::atomic<short>   size_2;
+    volatile std::atomic<int>     size_4;
+    volatile std::atomic<int64_t> size_8;
+
+    ++size_1;
+    ++size_2;
+    ++size_4;
+    ++size_8;
+
+    (void)size_1.load(std::memory_order_relaxed);
+    (void)size_2.load(std::memory_order_relaxed);
+    (void)size_4.load(std::memory_order_relaxed);
+    (void)size_8.load(std::memory_order_relaxed);
+
     return 0;
 }")
 
 check_cxx_source_compiles("${atomic_test_sources}" HAVE_STDATOMIC)
 if(NOT HAVE_STDATOMIC)
     set(_req_libraries "${CMAKE_REQUIRED_LIBRARIES}")
-    set(CMAKE_REQUIRED_LIBRARIES "atomic")
+    find_library(atomic_LIB atomic REQUIRED)
+    set(CMAKE_REQUIRED_LIBRARIES ${atomic_LIB})
     check_cxx_source_compiles("${atomic_test_sources}" HAVE_STDATOMIC_WITH_LIB)
     set(CMAKE_REQUIRED_LIBRARIES "${_req_libraries}")
 endif()
 
 add_library(WrapAtomic::WrapAtomic INTERFACE IMPORTED)
 if(HAVE_STDATOMIC_WITH_LIB)
-    target_link_libraries(WrapAtomic::WrapAtomic INTERFACE atomic)
+    # atomic_LIB is already found above.
+    target_link_libraries(WrapAtomic::WrapAtomic INTERFACE ${atomic_LIB})
 endif()
 
 set(WrapAtomic_FOUND 1)