[ATfE] Throw an error if a requested variant name is not found (#457) (#465)

By default the toolchain enables all available library variants to be
built, but a list of specific variants can instead be enabled through
the `-DLLVM_TOOLCHAIN_LIBRARY_VARIANTS` option. Currently, if an invalid
name is specified, the configuration will not notice and that variant
will silently not be built. This patch instead causes the script to
throw an error, so that it is clear that an expected variant is missing
if the name is incorrect.

(cherry picked from commit 2bce241)

Author: dcandler

arm-software/embedded/arm-multilib/CMakeLists.txt

@@ -173,6 +173,7 @@ foreach(lib_idx RANGE ${lib_count_dec})
     endif()
 
     if(variant IN_LIST ENABLE_VARIANTS OR ENABLE_VARIANTS STREQUAL "all")
+        list(APPEND enabled_variants ${variant})
         string(JSON variant_multilib_flags GET ${lib_def} "flags")
         # Placeholder libraries won't have a json, so store the error in
         # a variable so a fatal error isn't generated.
@@ -364,6 +365,16 @@ foreach(lib_idx RANGE ${lib_count_dec})
 
 endforeach()
 
+# Check that all variants that were configured to be enabled
+# were actually enabled.
+if(NOT ENABLE_VARIANTS STREQUAL "all")
+    foreach(expected_variant ${ENABLE_VARIANTS})
+        if(NOT expected_variant IN_LIST enabled_variants)
+            message(FATAL_ERROR "Variant name ${expected_variant} not found in ${MULTILIB_JSON}")
+        endif()
+    endforeach()
+endif()
+
 # Multilib file is generated in two parts.
 # 1. Template is filled with multilib flags from json
 configure_file(