[Chapel Merge] Disable llvm for whitebox testing

Branch: refs/heads/master
Revision: cb97b1e
Author: ronawho
Log Message:

Merge pull request #17655 from ronawho/disable-llvm-whitebox

Disable llvm for whitebox testing

[reviewed by @daviditen]

Whitebox testing is explicitly trying to test with different backend C
compilers, so disable llvm testing here.

Modified Files:
M util/cron/common-whitebox.bash

Compare: https://github.com/chapel-lang/chapel/compare/e1d154becf7e...cb97b1e743b5