Do not strip symbols having the flag REFERENCED_DYNAMICALLY set.
This is compatible with the behavior of cctools' strip and consistent with the documentation ("Mach-O Runtime Architecture"):
"REFERENCED_DYNAMICALLY (0x10)—Must be set for any symbol that might be referenced
by another image. The strip tool uses this bit to avoid removing symbols that must exist:
If the symbol has this bit set, strip does not strip it."
Test plan: make check-all
It's probably worth a comment showing what this n_desc value represents.