autoconf: Don't AC_SUBST() LD in configure
The only use of $(LD) in Makefiles is for AIX, to generate the export file for
the backend. We only support the system linker on AIX and we already hardcode
the path to a number of other binaries. Removing LD substitution will simplify
the upcoming meson PGXS compatibility.
While at it, add a comment why -r is used.
A subsequent commit will remove the determination of LD from configure as
well.
Discussion: https://postgr.es/m/
20221005200710.luvw5evhwf6clig6@awork3.anarazel.de