fix: should always use manually-installed llvm, regardless of platform

[coolaj86]

This use of command prefixes is redundant since command -v clang is just looking up clang in the user's PATH - which is what typing the clang command will do already, except in situations where there's a shell function or alias called clang (which is not the case on macOS or Linux).

Also, clang must be installed via the official tarball regardless of macOS or Linux.

Issue being fixed or feature implemented

Just documenting behavior.

What was done?

Documented that llvm must be installed manually (regardless of mac vs linux).

Removed redundant (and potentially confusing) use of command -v to define a command that's already in the user's PATH.

How Has This Been Tested?

• [x] Ubuntu 22.04 (LTS)'s apt's clang won't build these tools, but the official version will.
  See https://github.com/dashpay/platform/issues/1712.
• [x] macOS 13.6.1
  (tested on a fresh install via UTM virtual machine)

Breaking Changes

N/A

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated relevant unit/integration/functional/e2e tests
• [ ] I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• [x] I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• [ ] I have assigned this pull request to a milestone

[coolaj86]

I finished testing on macOS to confirm that the AR and CC variables are NOT necessary (no xcode magic alias trickery).

[shumkov]

@coolaj86 please reopen PR and update the branch if this is still actual