[lld-macho][reland] Add support for emitting dylibs with a single symbol

This got reverted due to UBSAN errors in a diff lower in the stack,
which is being fixed in https://reviews.llvm.org/D79050. This diff is
otherwise identical to the original https://reviews.llvm.org/D76908
(which was committed in 9598778b and reverted in b52bc265).

Differential Revision: https://reviews.llvm.org/D79051