[CMake] Enable LLVM_ENABLE_PER_TARGET_RUNTIME_DIR by default on Linux

This makes the LLVM_ENABLE_PROJECTS mode (supported for compiler-rt, deprecated
(D112724) for libcxx/libcxxabi/libunwind) closer to
https://libcxx.llvm.org/BuildingLibcxx.html#bootstrapping-build .
The layout is arguably superior because different libraries of target triples
are in different directories, similar to GCC/Debian multiarch.

When LLVM_DEFAULT_TARGET_TRIPLE is x86_64-unknown-linux-gnu,
`lib/clang/15.0.0/lib/libclang_rt.asan-x86_64.a`
is moved to
`lib/clang/15.0.0/lib/x86_64-unknown-linux-gnu/libclang_rt.asan.a`.

In addition, if the host compiler supports -m32 (multilib),
`lib/clang/15.0.0/lib/libclang_rt.asan-i386.a`
is moved to
`lib/clang/15.0.0/lib/i386-unknown-linux-gnu/libclang_rt.asan.a`.

Reviewed By: mstorsjo, ldionne, #libc

Differential Revision: https://reviews.llvm.org/D107799