Update find_ld.m4 for more robust lld detection

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite.

I noticed ./configure doesn't detect lld on debian bullsesye, at least for lld-13 which I tried. I also noticed the lld-13 package (again from https://apt.llvm.org/) doesn't create ld.lld. I've made a symlink, but not sure if this script should attempt to find it without that.