Fix xilinx_xrt() root discovery on Debian multiarch installs

[wendyliang25]

xilinx_xrt() derived the XRT root by taking two parent_path() levels up from the libxrt_coreutil.so path reported by dladdr(). This assumes coreutil lives directly in /lib, which only holds when CMAKE_INSTALL_LIBDIR is a single path component (lib, lib64). On a Debian multiarch install CMAKE_INSTALL_LIBDIR is lib/x86_64-linux-gnu, so coreutil is one level deeper and the two-level strip returns /lib instead of . shim_path() then re-appends XRT_LIB_DIR, producing a doubled path such as
'/opt/amdgpu/lib/lib/x86_64-linux-gnu/libxrt_core.so.2' and the load fails with "No such library".

Strip exactly as many trailing components as XRT_LIB_DIR contains instead of a hardcoded two levels, so the root is recovered correctly for lib, lib64, and lib/x86_64-linux-gnu layouts alike. Single-component layouts (standard, lib64, single-arch Yocto) are unaffected. Also default XRT_LIB_DIR to "lib" so the header stays self-contained; the loop additionally tolerates an empty value.

Regression from 11169ba (SWSPLAT-24084) which switched XRT root discovery from XILINX_XRT to dladdr().

Problem solved by the commit

Bug / issue (if any) fixed, which PR introduced the bug, how it was discovered

How problem was solved, alternative solutions (if any) and why they were rejected

Risks (if any) associated the changes in the commit

What has been tested and how, request additional testing if necessary

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[Copilot]

Pull request overview

This PR fixes Linux XRT install-root discovery when XRT is installed using Debian multiarch layouts (e.g. lib/x86_64-linux-gnu). It updates xilinx_xrt() to derive the root by stripping the number of path components in XRT_LIB_DIR rather than assuming a fixed .../lib/... depth, avoiding doubled-path construction that breaks module loading.

Changes:

• Add a default for XRT_LIB_DIR and document multiarch library layouts.
• Update xilinx_xrt() to strip as many directory components as XRT_LIB_DIR contains when deriving the XRT root.

Suppressed comments (1)

src/runtime_src/core/common/detail/linux/xilinx_xrt.h:79

• This loop currently uses XRT_LIB_DIR directly; if you switch to a local fallback (see earlier comment), update the loop to use that value so the code doesn’t rely on the macro being defined by this header.

    for (const auto& comp : sfs::path(XRT_LIB_DIR)) {

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[github-actions]

clang-tidy made some suggestions

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[stsoe]

Yeah, true, the code was wrong. Thanks for fixing.
A few suggestions, one being a corner case.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"