[llvm-libgcc][CMake] Refactor llvm-libgcc (#65455)

There are some issues in `llvm-libgcc` before this patch:

Commit c5a20b5182 ([llvm-libgcc] initial
commit)
uses `$<TARGET_OBJECTS:unwind_static>` to get libunwind objects, which
is empty.
The built library is actually a shared version of libclang_rt.builtins.

When configuring with `llvm/CMakeLists.txt`, target `llvm-libgcc`
requires a
corresponding target in `llvm-libgcc/CMakeLists.txt`.

Per target installation is not handled by `llvm-libgcc`, which is not
consistent
with `libunwind`.


This patch fixes those issues by:

Reusing target `unwind_shared` in `libunwind`, linking
`compiler-rt.builtins`
objects into it, and applying version script.

Adding target `llvm-libgcc`, creating symlinks, and utilizing cmake's
dependency
and component mechanism to ensure link targets will be built and
installed along
with symlinks.

Mimicking `libunwind` to handle per target installation.


It is quite hard to set necessary options without further modifying the
order of
runtime projects in `runtimes/CMakeLists.txt`. So though this patch
reveals the
possibility of co-existence of `llvm-libgcc` and
`compiler-rt`/`libunwind` in
`LLVM_ENABLE_RUNTIMES`, we still inhibit it to minimize influence on
other
projects, considering that `llvm-libgcc` is only intended for toolchain
vendors,
and not for casual use.

runtimes/CMakeLists.txt

@@ -216,23 +216,6 @@ if(LLVM_INCLUDE_TESTS)
   umbrella_lit_testsuite_begin(check-runtimes)
 endif()
 
-# llvm-libgcc incorporates both compiler-rt and libunwind as subprojects with very
-# specific flags, which causes clashes when they're independently built too.
-if("llvm-libgcc" IN_LIST runtimes)
-  if("compiler-rt" IN_LIST runtimes OR "compiler-rt" IN_LIST LLVM_ENABLE_PROJECTS)
-    message(FATAL_ERROR
-      "Attempting to build both compiler-rt and llvm-libgcc will cause irreconcilable "
-      "target clashes. Please choose one or the other, but not both.")
-  endif()
-
-  if("libunwind" IN_LIST runtimes)
-    message(
-      FATAL_ERROR
-      "Attempting to build both libunwind and llvm-libgcc will cause irreconcilable "
-      "target clashes. Please choose one or the other, but not both.")
-  endif()
-endif()
-
 # We do this in two loops so that HAVE_* is set for each runtime before the
 # other runtimes are added.
 foreach(entry ${runtimes})