Ensure lld respects the WITH/WITHOUT_SHARED_TOOLCHAIN option

Traditionally, toolchain components such as cc, as, and ld have been
built as static executables. The WITH_SHARED_TOOLCHAIN option from
src.conf(5) is meant to link these as regular executables, e.g. using
shared libraries.

The build of ld.lld did not yet check this option. Fix the Makefile so
it will do so now.

Reported by: Mike Cui <cuicui@gmail.com>
PR: 241257
MFC after: 3 days