[mach-o] Add support for using export tries

On Darwin at runtime, dyld will prefer to use the export trie of a dylib instead
of the traditional symbol table (which is large and requires a binary search).

This change enables the linker to generate an export trie and to prefer it if
found in a dylib being linked against.  This also simples the yaml for dylibs
because the yaml form of the trie can be reduced to just a sequence of names.

llvm-svn: 217066