Multilib Library path.

[CRKatri]

In order to improve cross compilation ability we could setup multilib library paths. Because MacOS and iOS share triples though, we may want to use the MEMO_TARGET. To make this work well, we would set libdir to $(MEMO_PREFIX)$(MEMO_SUB_PREFIX)/lib/$(MEMO_TARGET), and add that path to the end of DYLD_FALLBACK_LIBRARY_PATH (dyld(1)), so that if the library isn't found in the normal path it will check the multilib location. This is similar to how Debian does it. One issue with this is that MacOS targets install to /opt/procursus so it may be impossible for iOS libs installed on MacOS, but for installing arm64e and arm64 on MacOS it would work well, and AppleTV/WatchOS on iOS and vice versa.

[Diatrus]

This is a great idea I wanted to do for a long time, but not super feasible when it comes to the workload required for this.

[CRKatri]

It shouldn't be that big a work load.

1. Add DYLD_FALLBACK_PATH to profile.d.
2. Set libdir to the new path in DEFAULT_*_FLAGS
3. Libraries will use the new path as packages are updated.

[CRKatri]

@Diatrus I just push ck/multilib-experiment, which has some starter stuff, but it still needs a lot of work, it didn't occur to me that the *-package targets hard code $(MEMO_PREFIX)$(MEMO_SUB_PREFIX)/lib/libwhatever when packaging. It may be better to do this during the rewrite that I've been hearing about.

[CRKatri]

FYI: DYLD_FALLBACK_LIBRARY_PATH is ignored for setuid binaries on MacOS (possibly iOS too, but the section to prune them is in an if block for TARGET_OS_OSX (src/dyld2.cpp)) so any package with setuid bins may need some special depends on libraries that are multilib'd.