After the clang/llvm version 11 import LLD_VERSION is no longer used
upstream so Version.inc now only defines LLD_VERSION_STRING.

This breaks the WANT_LINKER_VERSION magic and might lead to us building
more than needed (e.g., for croos-tools).

Change the awk script to parse LLD_VERSION_STRING instead of LLD_VERSION,
which not only unbreaks the current situation but should also be backwards
compatible as dim points out.

Merging this before r364284 will ensure that stable/12 won't break.

PR:		[[ https://bugs.freebsd.org/248818 | 248818 ]]
Reviewed by:	emaste, dim (seems right and the way to go)