Update cmake/BuildTVM.cmake to use system llvm

[leezu]

Fixes #18229

Just now it's failing again with

-- Prepare external packages for TVM...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100   177  100   177    0     0  11800      0 --:--:-- --:--:-- --:--:-- 29500
tar: This does not look like a tar archive
xz: (stdin): File format not recognized
tar: Child returned status 1
tar: Error is not recoverable: exiting now

[mxnet-bot]

Hey @leezu , Thanks for submitting the PR
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