[llvm] [libsycl] Move and rename the shared library (PR #188770)

[Yury Plyakhin via llvm-commits]

YuriPlyakhin wrote:

> @tahonermann It's the other way around, the change makes the install location of the library dependent on `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR`, while removing that dependency for the build location. So we go from "build/lib/x86_64-unknown-linux-gnu/libsycl.so" to "build/lib/libLLVMSYCL.so", and from "install/lib/libsycl.so" to "install/lib/x86_64-unknown-linux-gnu/libLLVMSYCL.so", which is how the other runtimes seem to handle it.

@sergey-semenov , looking at how it is done for libcxx, it seems both build location and install location are using the target-dependend location:
https://github.com/llvm/llvm-project/blob/e56161ef9841d61ea06e6b0a79e195e9a84addb1/libcxx/CMakeLists.txt#L457
https://github.com/llvm/llvm-project/blob/e56161ef9841d61ea06e6b0a79e195e9a84addb1/libcxx/CMakeLists.txt#L461
when `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` is `ON`.

https://github.com/llvm/llvm-project/pull/188770