Pass through unknown architectures

Conan supports non-standard architectures through the installation of
custom settings.yml files. However, conan-cmake currently expects to be
able to translate the architecture from the CMake naming scheme to the
Conan naming scheme, and fails with an error if the architecture is
unknown.

This patch resolves the issue by making the script simply pass through
the CMake architecture to Conan unchanged in these cases.

conan_provider.cmake

@@ -83,6 +83,8 @@ function(detect_arch ARCH)
         set(_ARCH x86)
     elseif(host_arch MATCHES "AMD64|amd64|x86_64|x64")
         set(_ARCH x86_64)
+    else()
+        set(_ARCH "${CMAKE_SYSTEM_PROCESSOR}")
     endif()
     message(STATUS "CMake-Conan: cmake_system_processor=${_ARCH}")
     set(${ARCH} ${_ARCH} PARENT_SCOPE)