[build-script] Allow primary-variant-arch and primary-variant-sdk to be set independently.

It's unclear why they're grouped, and it's also confusing.

Author: Davide Italiano

utils/build-script-impl

@@ -1712,10 +1712,17 @@ for host in "${ALL_HOSTS[@]}"; do
                         -DSWIFT_SDKS:STRING="$(join ";" ${SWIFT_SDKS[@]})"
                     )
                 fi
+
                 if [[ "${SWIFT_PRIMARY_VARIANT_SDK}" ]] ; then
                     cmake_options=(
                         "${cmake_options[@]}"
                         -DSWIFT_PRIMARY_VARIANT_SDK:STRING="${SWIFT_PRIMARY_VARIANT_SDK}"
+                    )
+                fi
+
+                if [[ "${SWIFT_PRIMARY_VARIANT_ARCH}" ]] ; then
+                    cmake_options=(
+                        "${cmake_options[@]}"
                         -DSWIFT_PRIMARY_VARIANT_ARCH:STRING="${SWIFT_PRIMARY_VARIANT_ARCH}"
                     )
                 fi