linkage_checker: Add harmless dylibs check.

While the linkage checker should normally report all broken links,
there are a few cases where broken links are harmless. One case
is when a the PPC slice of a program links to a PPC-only library
that no longer exists on x86_64 OS X. Since the PPC slice is never
loaded on modern OS X, inconsistencies within it do not need to
be reported.

Library/Homebrew/os/mac/linkage_checker.rb

@@ -36,6 +36,7 @@ class LinkageChecker
           rescue NotAKegError
             @system_dylibs << dylib
           rescue Errno::ENOENT
+            next if harmless_broken_link?(dylib)
             @broken_dylibs << dylib
           else
             tap = Tab.for_keg(owner).tap
@@ -114,6 +115,14 @@ class LinkageChecker
 
   private
 
+  # Whether or not dylib is a harmless broken link, meaning that it's
+  # okay to skip (and not report) as broken.
+  def harmless_broken_link?(dylib)
+    # libgcc_s_ppc64 is referenced by programs that use the Java Service Wrapper,
+    # and is harmless on x86(_64) machines
+    ["/usr/lib/libgcc_s_ppc64.1.dylib"].include?(dylib)
+  end
+
   # Display a list of things.
   # Things may either be an array, or a hash of (label -> array)
   def display_items(label, things)