Update README for variable path references and fix LLVM_TARGETS_TO_BUILD

[FlyMan2004]

This pull request updates the README.md to improve portability and clarity by replacing hardcoded absolute paths with the ${LINX_ISA_ROOT} environment variable and correcting the LLVM target name.

Documentation improvements:

• Replaced all hardcoded absolute paths (e.g., /Users/zhoubot/linx-isa) with the ${LINX_ISA_ROOT} environment variable to make build and test instructions more portable.
• Updated LLVM build instructions to use the correct target name (LinxISA instead of Linx).

[github-actions]

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[gemini-code-assist]

Code Review

This pull request updates the README.md to replace hardcoded local file paths with the ${LINX_ISA_ROOT} environment variable and updates the LLVM target name to LinxISA. Feedback suggests that using the repository root is clearer for new users than an environment variable and recommends using relative paths for documentation links to ensure they remain clickable in Markdown viewers.

[gemini-code-assist]

The instruction to run from ${LINX_ISA_ROOT} assumes the environment variable is already defined. For a user who has just cloned the repository, this variable will not exist. It is clearer to instruct the user to run from the repository root.

Suggested change

	Run from `${LINX_ISA_ROOT}`.
	Run from the repository root.

[gemini-code-assist]

Using environment variables in the 'Related Docs' section prevents these paths from being rendered as clickable links in most Markdown viewers (e.g., GitHub, VS Code). Using relative paths is more idiomatic for repository documentation and ensures the links are functional.

Suggested change

	- `${LINX_ISA_ROOT}/docs/project/navigation.md`
	- `${LINX_ISA_ROOT}/docs/bringup/`
	- `${LINX_ISA_ROOT}/avs/compiler/linx-llvm/README.md`
	- [docs/project/navigation.md](docs/project/navigation.md)
	- [docs/bringup/](docs/bringup/)
	- [avs/compiler/linx-llvm/README.md](avs/compiler/linx-llvm/README.md)

[Copilot]

Pull request overview

This PR updates the top-level build/test documentation to be more portable by replacing developer-specific absolute paths with a ${LINX_ISA_ROOT} variable, and it attempts to correct the LLVM target name used in the CMake configure command.

Changes:

• Replaced hardcoded /Users/... paths in build/test instructions with ${LINX_ISA_ROOT}.
• Updated the documented LLVM_TARGETS_TO_BUILD value from Linx to LinxISA.
• Updated “Related Docs” paths to use ${LINX_ISA_ROOT}.