[clang] [llvm] [OpenEmbedded] Fix include and lib paths for multilib targets (PR #121302)

[Mike Crowe via llvm-commits]

mikecrowe wrote:

I [tried asking](https://lists.openembedded.org/g/openembedded-core/topic/gcc_library_directory_and/111849200) about this on the OpenEmbedded mailing list. The response was that it has been this way for so long that it's going to be a major undertaking to change it even though they would quite like to do so.

I had a go at using symlinks to work around the problem. Unfortunately because `ScanLibDirForGCCTriple` ends up using `../../..` to turn `/usr/lib/gcc/${TARGET_SYS}/${gccversion}` (where `${TARGET_SYS}` is the triple) back into just `/usr/lib` it's necessary to have real directories for each of those components and only have symlinks below that. In other words:
```sh
mkdir -p $sysroot/usr/lib/gcc/arm-oemllib32-linux-gnueabi/9.5.0
cd $sysroot/usr/lib/gcc/arm-oemllib32-linux-gnueabi/9.5.0
ln -s ln -s ../../../arm-oemllib32-linux-gnueabi/9.5.0/* .
```

Presumably there's a reason that `../../..` (made up of the `ReversePath` in the `Suffixes` array and another `/../` when constructing `GCCParentLibPath`) is used rather than removing components from the path or just using the `--sysroot` path to find `/usr/lib`?

https://github.com/llvm/llvm-project/pull/121302