Fix problems after LLVM configuration PRs

[mppf]

Follow-up to PRs #20396 and #20385 to address issues raised by nightly testing.

• build failures in cron-xc-wb-host-prgenv-cray-failure and xc-wb.prgenv-cray configurations: don't default to CHPL_LLVM_SUPPORT=system if all the LLVM dependencies are not installed. These configurations have just llvm-config but not the required headers.
• networking-packages failure with curl tests: update common-quickstart.bash to set CHPL_LLVM=none. The quickstart setup script can provide CHPL_LLVM=none or CHPL_LLVM=system but since we test CHPL_LLVM=system in most configurations, in the quickstart testing configuration, we want to test CHPL_LLVM=none. At the moment, the curl tests do not all work with the LLVM backend.
• darwin-m1 or other Mac OS testing failure with chpl-env-gen: fixed a sed portability issue between linux and Mac OS X.

Reviewed by @arezaii - thanks!

• On Ubuntu, if llvm-14 is installed but not llvm-14-dev
  • default is CHPL_LLVM=unset and CHPL_LLVM_SUPPORT=bundled
  • setting CHPL_LLVM=system produces an error from printchplenv about a missing header
• On Ubuntu, if we have the needed LLVM dependencies
  • default is CHPL_LLVM=system and CHPL_LLVM_SUPPORT=system
• make functions on XC with PrgEnv-cray and GCC as the host compiler
• make functions on XC with PrgEnv-cray and cray-prgenv-cray as the host compiler
• full local testing