configure: Silence lld-link when getting the version number

In recent lld-link versions, this command prints the version to
stdout, but also prints an error to stderr:

$ lld-link -flavor gnu --version
LLD 4.0.0 (trunk 285641)
lld-link: error: no input files
lld-link: error: target emulation unknown: -m or at least one .o file required

Signed-off-by: Martin Storsjö <martin@martin.st>