[Qt WebEngine] The logic of the CMake function `get_architectures` is incorrect

The CMake funcion `get_architectures` in `qtwebengine/cmake/Functions.cmake` is incorrect.
On Windows x64, the variable `${CMAKE_SYSTEM-PCESSOR}` always returns `AMD64`, even if MSVC target architecture is x86.
This results in many compilation errors when compiling qtwebengine for x86 architecture.
eg,
```
qtwebengine\src\3rdparty\chromium\base/allocator/partition_allocator/partition_alloc_config.h(32): error C2338: static_assert failed: ''
qtwebengine\src\3rdparty\chromium\base/allocator/partition_allocator/partition_address_space.h(274): error C2607: static assertion failed
qtwebengine\src\3rdparty\chromium\base/allocator/partition_allocator/partition_address_space.h(275): error C2607: static assertion failed
qtwebengine\src\3rdparty\chromium\base/allocator/partition_allocator/partition_address_space.h(282): error C2607: static assertion failed
qtwebengine\src\3rdparty\chromium\base/allocator/partition_allocator/partition_address_space.h(283): error C2607: static assertion failed
```

Reference from website:
https://cmake.org/cmake/help/latest/variable/CMAKE_SYSTEM_PROCESSOR.html
https://gitlab.kitware.com/cmake/cmake/-/issues/15170#note_891234