Make lld version check more robust

[eleanor-arm]

Modified or downstream versions of lld might extend the --version
output to include further lines of information. This change tightens
up the sed command so that it will only extract a version number
appearing after the name "LLD"; quit after printing the first one it
finds; and not emit additional lines of --version text that don't
include a version number at all.

"Contribution covered by the CLA"

[admlck]

Thank you. The change has been reviewed and merged internally.