install: place the library in architecture-specific directories on ELF platforms

This is needed for swiftlang/swift#63782, which changes the Unix toolchain to look
for libraries in architecture-specific directories.

Author: finagolfin

CMakeLists.txt

@@ -116,9 +116,13 @@ endif()
 
 set_property(GLOBAL APPEND PROPERTY XCTest_EXPORTS XCTest)
 get_swift_host_arch(swift_arch)
+set(SWIFT_INSTALL_SUBDIR "$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>")
+if(NOT CMAKE_SYSTEM_NAME MATCHES "Darwin|Windows")
+  set(SWIFT_INSTALL_SUBDIR "${SWIFT_INSTALL_SUBDIR}/${swift_arch}")
+endif()
 install(TARGETS XCTest
-  ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>
-  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>
+  ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/${SWIFT_INSTALL_SUBDIR}
+  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/${SWIFT_INSTALL_SUBDIR}
   RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
 install(FILES
   ${CMAKE_CURRENT_BINARY_DIR}/swift/XCTest.swiftdoc