[lld/mac] Implement support for -mark_dead_strippable_dylib

lld doesn't read MH_DEAD_STRIPPABLE_DYLIB to strip dead dylibs yet,
but now it can produce dylibs with it set.

While here, also switch an existing test that looks only at the main Mach-O
header from --all-headers to --private-header.

Differential Revision: https://reviews.llvm.org/D98262