kbuild: Fix ld-version.sh script if LLD was built with LLD_VENDOR

If LLD was built with -DLLD_VENDOR="xyz", ld.lld --version output
will prefix LLD_VENDOR. Since LLD_VENDOR can contain spaces, the
LLD identifier isn't guaranteed to be $2 either.

Adjust the version checker to handle such versions of lld.

Link: https://lore.kernel.org/lkml/20210302221211.1620858-1-bero@lindev.ch/
Signed-off-by: Bernhard Rosenkränzer <bero@lindev.ch>
[masahiro yamada: refactor the code]
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>