[OpenMP] Respect LLVM per-target install directories (#83282)

Summary:
One recurring problem we have with the OpenMP libraries is that they are
potentially conflicting with ones found on the system, this occurs when
there are two copies and one is used for linking that it not attached to
the correspoding clang compiler. LLVM already uses target specific
directories for this, like with libc++, which are always searched first.
This patch changes the install directory to be
`lib/x86_64-unknown-linux-gnu` for example.

Notable changes would be that users will need to change their
LD_LIBRARY_PATH settings optionally, or use default rt-rpath options.
This should fix problems were users are linking the wrong versions of
static libraries