[lld-macho] Support -bundle

Not 100% sure but it appears that bundles are almost identical to
dylibs, aside from the fact that they do not contain `LC_ID_DYLIB`. ld64's code
seems to treat bundles and dylibs identically in most places.

Supporting bundles allows us to run e.g. XCTests, as all test suites are
compiled into bundles which get dynamically loaded by the `xctest` test runner.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D87856