llvmPackages_15, llvmPackages_16: Make the pkgsLLVM.stdenv work

What changed:

* Fixed crtbeginS.o and crtendS.o missing
  (they may or may not be called crt{begin_end},{,_shared}.

* Fixed implicit function declaration causing build errors for various
  builds by supplying -Wno-implicit-function-declaration.

* Fixed __cxxabi_config.h missing, by adding -I${cxxabi}/include/c++/v1
  in the wrapper.

* Fixed libcxx failing to build due to missing libunwind symbols by
  including libunwind as a buildInput, and setting
  -DLIBCXX_ADDITIONAL_LIBRARIES=unwind for stdenv.hostPlatform.useLLVM == true.

* libcxxabi wants to find libunwind at libunwind_shared.so, so symlink
  it there in libunwind.

* llvmPackages_16.libcxxabi: Pass -nostdlib via CMAKE_*_LINKER_FLAGS

  Without this flag, the link of libcxxabi.so tries to pull in libgcc and
  friends, from the clang compiler driver.

* Drop unneeded musl hack patch from libcxx.

* Pass -Wno-error=implicit-function-declaration only to compiler-rt

  See LLVM forum discussion:

  https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213

  In summary, LLVM 16 made implicit function declaration an error. This
  happens a lot in configure scripts which can break things.

* llvmPackages_16: !isDarwin: Supply -DLIBCXX_ABI_USE_LLVM_UNWINDER=On

  Otherwise it fails with various undefined references to _Unwind_*
  functions: (full list: _Unwind_DeleteException _Unwind_GetIP
  _Unwind_GetLanguageSpecificData _Unwind_GetRegionStart
  _Unwind_RaiseException _Unwind_Resume _Unwind_SetGR _Unwind_SetIP).

* 16.libcxxabi: Only pass -nostdlib for useLLVM and Darwin builds

What was tested:

* x86_64-linux, aarch64-linux, the stdenv builds.
  * Additionally I was able to get nix to build, with an overlay to fix
    a couple of minor issues in downstream packages (overlay supplied in
    PR #246577.

* aarch64-darwin fails spuriously in a single LLVM test
  strip-preserve-atime.test checking atime timestamps.

* The same for pkgsLLVM with llvmPackages = llvmPackages_15.

Signed-off-by: Peter Waller <p@pwaller.net>

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