llvmPackages: Fix more inconsistencies

The main thing was using `llvm_meta` in all versions.

Secondarily:

 - libunwindx7: Forgot to split outputs

 - libcxx{,abi} 12: Forgot to apply output-splitting patches.

 - simplify `useLLVM` stdenv-switching logic.

 - openmp always gets its own directory

pkgs/development/compilers/llvm/12/libcxxabi/gnu-install-dirs.patch

@@ -0,0 +1,34 @@
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 426c855288fc..a9812a994f53 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -27,6 +27,8 @@ set(LIBCXXABI_LIBCXX_PATH "${CMAKE_CURRENT_LIST_DIR}/../libcxx" CACHE PATH
+ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBCXXABI_STANDALONE_BUILD)
+   project(libcxxabi CXX C)
+ 
++  include(GNUInstallDirs)
++
+   set(PACKAGE_NAME libcxxabi)
+   set(PACKAGE_VERSION 11.0.0)
+   set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
+@@ -180,17 +182,17 @@ set(CMAKE_MODULE_PATH
+ 
+ if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
+   set(LIBCXXABI_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++)
+-  set(LIBCXXABI_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++)
++  set(LIBCXXABI_INSTALL_LIBRARY_DIR ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++)
+   if(LIBCXX_LIBDIR_SUBDIR)
+     string(APPEND LIBCXXABI_LIBRARY_DIR /${LIBCXXABI_LIBDIR_SUBDIR})
+     string(APPEND LIBCXXABI_INSTALL_LIBRARY_DIR /${LIBCXXABI_LIBDIR_SUBDIR})
+   endif()
+ elseif(LLVM_LIBRARY_OUTPUT_INTDIR)
+   set(LIBCXXABI_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
+-  set(LIBCXXABI_INSTALL_LIBRARY_DIR lib${LIBCXXABI_LIBDIR_SUFFIX})
++  set(LIBCXXABI_INSTALL_LIBRARY_DIR ${CMAKE_INSTALL_LIBDIR}${LIBCXXABI_LIBDIR_SUFFIX})
+ else()
+   set(LIBCXXABI_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXXABI_LIBDIR_SUFFIX})
+-  set(LIBCXXABI_INSTALL_LIBRARY_DIR lib${LIBCXXABI_LIBDIR_SUFFIX})
++  set(LIBCXXABI_INSTALL_LIBRARY_DIR ${CMAKE_INSTALL_LIBDIR}${LIBCXXABI_LIBDIR_SUFFIX})
+ endif()
+ 
+ set(LIBCXXABI_INSTALL_PREFIX "" CACHE STRING "Define libc++abi destination prefix.")