if(CMAKE_SYSTEM_PROCESSOR STREQUAL "aarch64")
    add_library(arch aarch64.cpp)
elseif(CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64" OR
       CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64le")
    add_library(arch ppc64.cpp)
elseif(CMAKE_SYSTEM_PROCESSOR STREQUAL "s390" OR
       CMAKE_SYSTEM_PROCESSOR STREQUAL "s390x")
    add_library(arch s390.cpp)
elseif(CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64")
    add_library(arch x86_64.cpp)
else()
  message(FATAL_ERROR "Unsupported architecture: {CMAKE_SYSTEM_PROCESSOR}")
endif()

if(CMAKE_SYSTEM_PROCESSOR STREQUAL "aarch64")
    add_library(arch aarch64.cpp)
elseif(CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64" OR
       CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64le")
    add_library(arch ppc64.cpp)
elseif(CMAKE_SYSTEM_PROCESSOR STREQUAL "s390" OR
       CMAKE_SYSTEM_PROCESSOR STREQUAL "s390x")
    add_library(arch s390.cpp)
elseif(CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64")
    add_library(arch x86_64.cpp)
else()
  message(FATAL_ERROR "Unsupported architecture: ${CMAKE_SYSTEM_PROCESSOR}")
endif()

Fix "Unsupported architecture" error message
Fix "Unsupported architecture" error message

To show a CMAKE variable value, a '$' prefix should be used.
