Prefer the new flag -lld-allow-duplicate-weak (which was added recently
in a67ae8c0fd), over the old -lldmingw.
The -lldmingw option enables a number of different behaviours, while
this test only is interested in one aspect of them.
This should allow making -lldmingw more strict with not enabling MSVC
specific behaviours like inferring lib directories automatically from
the environment, as being pursued in https://reviews.llvm.org/D144084.
8822eaae0f